diff --git a/layout-custom.c b/layout-custom.c
index 7c9c68bdc..ada162055 100644
--- a/layout-custom.c
+++ b/layout-custom.c
@@ -25,6 +25,63 @@
 
 #include "tmux.h"
 
+/*
+ * Layout string syntax
+ * --------------------
+ *
+ * A layout has one outer cell and may have a four-digit hexadecimal checksum:
+ *
+ *     layout = [checksum,](v2:new-cell | old-cell)
+ *
+ * The checksum covers all non-whitespace text after its comma, including the
+ * version. Checksums before nested cells are also accepted, although they are
+ * not generated. The v2: prefix is accepted only at the outer level and
+ * requires the new cell form. The new cell form requires v2: and unknown
+ * versions are rejected.
+ *
+ * New form:
+ *
+ *     geometry  = widthxheight[x-position[y-position]]
+ *     container = geometry{new-cell;new-cell;...}
+ *               | geometry[new-cell;new-cell;...]
+ *     pane      = %pane-id,z-index:geometry[:flags]
+ *     new-cell  = container | pane
+ *
+ * Braces arrange children from left to right and square brackets from top to
+ * bottom. Semicolons separate children. Positions use X geometry: +N and ++N
+ * are absolute, +-N is negative, and -N places the right or bottom edge N
+ * cells from the right or bottom. A missing position is +0; if only one is
+ * present, the second is +0. Relative right or bottom positions are permitted
+ * only for floating panes. Widths and heights are between PANE_MINIMUM and
+ * PANE_MAXIMUM; resolved positions are between -PANE_MAXIMUM and
+ * PANE_MAXIMUM.
+ *
+ * Pane IDs must be unique. Z-indexes must be unique and contiguous from zero,
+ * with floating panes before tiled panes. Flags are f (floating), h (hidden)
+ * and z (zoomed); h and z cannot be combined. Hidden panes contain their
+ * restoration geometry.
+ *
+ * Old form:
+ *
+ *     geometry = widthxheight,x-position,y-position
+ *     old-cell = geometry{old-cell,old-cell,...}
+ *              | geometry[old-cell,old-cell,...]
+ *              | geometry[,pane-id]
+ *
+ * Old pane IDs are optional and ignored. Old panes are tiled, visible and
+ * unzoomed. A layout must not mix the two cell forms or their separators.
+ * Whitespace may appear between tokens and is ignored by the checksum.
+ *
+ * Layouts rearrange existing panes; they do not create panes. New pane IDs are
+ * used when all match the target window, tree order is used when none match,
+ * and a partial match is rejected. Surplus cells are removed from the end when
+ * the target has fewer panes; a target with more panes is rejected.
+ *
+ * cmd-select-layout.c also accepts multiple window records in this form:
+ *
+ *     @window-id:layout [@window-id:layout ...]
+ */
+
 #define LAYOUT_STRING_MAX 8192
 #define LAYOUT_DEPTH_MAX 64
 
@@ -949,7 +1006,8 @@ layout_prepare(struct window *w, const char *layout, char **cause)
 		layout_free_cell(root, 0);
 		return (NULL);
 	}
-	if (version == 1 && ctx.format != LAYOUT_FORMAT) {
+	if ((version == 1 && ctx.format != LAYOUT_FORMAT) ||
+	    (version == 0 && ctx.format == LAYOUT_FORMAT)) {
 		*cause = xstrdup("invalid layout version");
 		layout_free_cell(root, 0);
 		return (NULL);
diff --git a/regress/layout-custom.sh b/regress/layout-custom.sh
index a1bf6d15d..757a32359 100755
--- a/regress/layout-custom.sh
+++ b/regress/layout-custom.sh
@@ -53,10 +53,9 @@ layout=$($TMUX display-message -p '#{window_layout}')
 must_equal "$layout" \
     'cc3c,v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
 
-# Both current and legacy layouts may omit the outer checksum.
-$TMUX select-layout '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}' || \
-    fail "checksumless current layout was rejected"
-must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
+# Checksums are optional, but the new form requires its version.
+must_fail $TMUX select-layout \
+    '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
 $TMUX select-layout \
     'v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}' || \
     fail "checksumless versioned layout was rejected"
@@ -109,8 +108,8 @@ $TMUX resize-pane -t %0 -Z || fail "tiled unzoom failed"
 $TMUX select-layout "$layout" || fail "layout could not be restored"
 
 # Whitespace and newlines between tokens do not affect the checksum.
-indented='93fc,
-  80x24 +0 +0 {
+indented='cc3c,
+  v2: 80x24 +0 +0 {
     %0, 0: 39x24 +0 +0;
     %1, 1: 40x24 +40 +0
   }'
@@ -137,12 +136,12 @@ $TMUX select-layout "$legacy" || fail "legacy layout could not be reapplied"
 layout=$($TMUX display-message -p '#{window_layout}')
 
 # A checksum may be attached to each nested cell but is not emitted.
-nested='73a1,80x24+0+0{6806,%0,0:39x24+0+0;6bda,%1,1:40x24+40+0}'
+nested='v2:80x24+0+0{6806,%0,0:39x24+0+0;6bda,%1,1:40x24+40+0}'
 $TMUX select-layout "$nested" || fail "nested checksums were rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
 
 # A bad nested checksum is rejected without changing the current layout.
-badnested='2fa4,80x24+0+0{dead,%0,0:39x24+0+0;6bda,%1,1:40x24+40+0}'
+badnested='v2:80x24+0+0{dead,%0,0:39x24+0+0;6bda,%1,1:40x24+40+0}'
 must_fail $TMUX select-layout "$badnested"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
 
@@ -152,23 +151,23 @@ $TMUX select-layout "$hidden" || fail "hidden layout was rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$hidden"
 
 # Pane IDs map by identity when all match and by tree order when none match.
-disjoint='80x24+0+0{%100,0:39x24+0+0;%101,1:40x24+40+0}'
+disjoint='v2:80x24+0+0{%100,0:39x24+0+0;%101,1:40x24+40+0}'
 $TMUX select-layout "$disjoint" || fail "disjoint pane IDs were rejected"
 must_equal "$($TMUX display-message -p '#{window_layout}')" "$layout"
-identity='80x24+0+0{%1,0:39x24+0+0;%0,1:40x24+40+0}'
+identity='v2:80x24+0+0{%1,0:39x24+0+0;%0,1:40x24+40+0}'
 $TMUX select-layout "$identity" || fail "matching pane IDs were rejected"
 must_equal "$($TMUX display-message -p -t %0 '#{pane_left}')" 40
 $TMUX select-layout "$legacy" || fail "legacy layout could not be restored"
 
 # Duplicate or partially matching IDs fail when pane counts match.
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0;%100,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;%100,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%100,0:39x24+0+0;%100,1:40x24+40+0}'
+    'v2:80x24+0+0{%100,0:39x24+0+0;%100,1:40x24+40+0}'
 
 # A target with fewer panes drops cells from the end, compacts z-indexes and
 # assigns the remaining cells in tree order. A target with more panes fails.
-fewer='80x24+0+0{%100,1:26x24+0+0;%101,2:26x24+27+0;%102,0:26x24+54+0:z}'
+fewer='v2:80x24+0+0{%100,1:26x24+0+0;%101,2:26x24+27+0;%102,0:26x24+54+0:z}'
 $TMUX select-layout "$fewer" || fail "surplus current cells were not removed"
 must_equal "$($TMUX display-message -p '#{window_panes}')" 2
 must_equal "$($TMUX display-message -p -t %0 '#{pane_left},#{pane_width}')" \
@@ -183,40 +182,40 @@ esac
 # The complete tree must be structurally valid before surplus cells are
 # removed; the last child has the wrong height here.
 must_fail $TMUX select-layout \
-    '80x24+0+0{%100,0:39x24+0+0;%101,1:20x24+40+0;%102,2:19x10+61+0}'
-must_fail $TMUX select-layout '%100,0:80x24+0+0'
+    'v2:80x24+0+0{%100,0:39x24+0+0;%101,1:20x24+40+0;%102,2:19x10+61+0}'
+must_fail $TMUX select-layout 'v2:%100,0:80x24+0+0'
 must_fail $TMUX select-layout '80x24,0,0,0'
 $TMUX select-layout "$legacy" || fail "legacy layout could not be restored"
 
 # Mixed legacy and new syntax, malformed containers, duplicate z-indices,
 # non-contiguous z-indices, invalid z-ordering and unknown flags fail.
 must_fail $TMUX select-layout \
-    '93ed,80x24+0+0{%0,0:39x24+0+0,%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0,%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
     '80x24,0,0{39x24,0,0,0;%1,1:40x24+40+0}'
-must_fail $TMUX select-layout '80x24+0+0{}'
+must_fail $TMUX select-layout 'v2:80x24+0+0{}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0;}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0|%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0|%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '93ec,80x24+0+0{%0,0:39x24+0+0;%1,0:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;%1,0:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0;%1,2:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;%1,2:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0:f}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0:f}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:38x24+0+0;%1,1:40x24+39+0}'
+    'v2:80x24+0+0{%0,0:38x24+0+0;%1,1:40x24+39+0}'
 must_fail $TMUX select-layout \
-    '0e42,80x24+0+0{%0,0:39x24+0+0:q;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0:q;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0:ff;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0:ff;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0:hh;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0:hh;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0:zz;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0:zz;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0:z;%1,1:40x24+40+0:z}'
+    'v2:80x24+0+0{%0,0:39x24+0+0:z;%1,1:40x24+40+0:z}'
 must_fail $TMUX select-layout \
     '80x24,0,0{39x24,0,0,0,40x24,40,0,}'
 
@@ -225,22 +224,22 @@ must_fail $TMUX select-layout '0000,'
 must_fail $TMUX select-layout \
     'c523,999999999999999999999999x24,0,0,0'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:0x24+0+0;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:0x24+0+0;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+2147483647+0;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+2147483647+0;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+-10001+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+-10001+0}'
 must_fail $TMUX select-layout \
-    '80x24-1+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+    'v2:80x24-1+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24-1+0;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24-1+0;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24+0+;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24+0+;%1,1:40x24+40+0}'
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:39x24++ +0;%1,1:40x24+40+0}'
-long='80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:39x24++ +0;%1,1:40x24+40+0}'
+long='v2:80x24+0+0{%0,0:39x24+0+0;%1,1:40x24+40+0}'
 i=0
 while [ $i -lt 9000 ]; do
 	long="${long}x"
@@ -252,7 +251,7 @@ $TMUX list-windows >/dev/null || fail "server exited after invalid layouts"
 # Invalid custom layouts must not unzoom the target window.
 $TMUX resize-pane -t %0 -Z || fail "zoom failed"
 must_fail $TMUX select-layout \
-    '80x24+0+0{%0,0:0x24+0+0;%1,1:40x24+40+0}'
+    'v2:80x24+0+0{%0,0:0x24+0+0;%1,1:40x24+40+0}'
 must_equal "$($TMUX display-message -p -t %0 '#{pane_zoomed_flag}')" 1
 $TMUX resize-pane -t %0 -Z || fail "unzoom failed"
 
@@ -269,8 +268,8 @@ while [ $i -lt 64 ]; do
 	deep="1x1+0+0{$deep}"
 	i=$((i + 1))
 done
-$TMUX select-layout "$deep" || fail "maximum layout depth was rejected"
-must_fail $TMUX select-layout "1x1+0+0{$deep}"
+$TMUX select-layout "v2:$deep" || fail "maximum layout depth was rejected"
+must_fail $TMUX select-layout "v2:1x1+0+0{$deep}"
 
 # X-style geometry supports right/bottom-relative offsets, absolute negative
 # offsets, doubled plus signs, and omitted positions.
@@ -289,14 +288,14 @@ case "$($TMUX display-message -p '#{window_layout}')" in
 *) fail "negative floating pane was not serialized with +- offsets" ;;
 esac
 
-relative='1442,120x40+0+0{%0,1:120x40+0+0;%1,0:30x10-10-20:f}'
+relative='v2:120x40+0+0{%0,1:120x40+0+0;%1,0:30x10-10-20:f}'
 $TMUX select-layout "$relative" || fail "relative geometry was rejected"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '80,10'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}')"
 
-bottomright='120x40+0+0{%0,1:120x40+0+0;%1,0:30x10-0-0:f}'
+bottomright='v2:120x40+0+0{%0,1:120x40+0+0;%1,0:30x10-0-0:f}'
 $TMUX select-layout "$bottomright" || fail "bottom-right geometry failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '90,30'
@@ -304,32 +303,32 @@ must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+90+30:f}')"
 
 absolute_body='120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:f}'
-$TMUX select-layout "3a22,$absolute_body" || \
+$TMUX select-layout "v2:$absolute_body" || \
     fail "absolute negative geometry failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top}')" \
     '-10,-20'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned "$absolute_body")"
 
-hiddenfloat='120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}'
+hiddenfloat='v2:120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}'
 $TMUX select-layout "$hiddenfloat" || fail "hidden floating layout failed"
 must_equal "$($TMUX display-message -p -t %1 '#{pane_left},#{pane_top},#{pane_width},#{pane_height}')" \
     '-10,-20,20,8'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}')"
 must_fail $TMUX select-layout \
-    '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fhz}'
+    'v2:120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fhz}'
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:20x8+-10+-20:fh}')"
 must_fail $TMUX select-layout \
-    '120x40+0+0{%0,1:120x40+0+0;%1,0:10000x8-10000+0:f}'
+    'v2:120x40+0+0{%0,1:120x40+0+0;%1,0:10000x8-10000+0:f}'
 
-defaults='fcb6,120x40{%0,1:120x40;%1,0:30x10++80+10:f}'
+defaults='v2:120x40{%0,1:120x40;%1,0:30x10++80+10:f}'
 $TMUX select-layout "$defaults" || fail "default or doubled offsets failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+10:f}')"
 
-oneoffset='8f93,120x40{%0,1:120x40;%1,0:30x10+80:f}'
+oneoffset='v2:120x40{%0,1:120x40;%1,0:30x10+80:f}'
 $TMUX select-layout "$oneoffset" || fail "single offset default failed"
 must_equal "$($TMUX display-message -p '#{window_layout}')" \
     "$(versioned '120x40+0+0{%0,1:120x40+0+0;%1,0:30x10+80+0:f}')"
diff --git a/tmux.1 b/tmux.1
index b5f7696c8..9117c73d6 100644
--- a/tmux.1
+++ b/tmux.1
@@ -3975,142 +3975,25 @@ applies the last set layout if possible (undoes the most recent layout change).
 spreads the current pane and any panes next to it out evenly.
 .Pp
 .Ar layout\-name
-may be a layout description with an optional outer checksum followed by one
-root layout cell:
-.Bd -literal -offset indent
-layout = [checksum,][v2:]cell
-.Ed
-.Pp
-The
+may be a layout description in either a new or old form, optionally beginning
+with a four-digit hexadecimal checksum and a comma.
+The new form is identified by
 .Ql v2:
-prefix identifies the new form and is included in generated layouts using the
-new form.
-It may be omitted when reading a layout in the new form for compatibility
-with older output.
-An explicit
-.Ql v2:
-prefix may not be used with the old cell form and unknown versions are
-rejected.
-.Pp
-A layout cell is either a container cell or a pane cell.
-Geometry gives a cell's size and position.
-.Nm
-accepts X Window System-style geometry specifications:
-.Bd -literal -offset indent
-geometry = widthxheight[x-offset[y-offset]]
-x-offset = +N | ++N | +-N | -N
-y-offset = +N | ++N | +-N | -N
-.Ed
-.Pp
-.Ql +N
-is an absolute offset from the left or top of the window.
-.Ql -N
-is accepted only for floating pane cells and places the pane's right or bottom
-edge relative to the right or bottom of the window.
-To specify a negative offset to place a floating pane off the left or above the
-top of the window, specify it as:
-.Ql +-N .
-.Ql ++N
-is equivalent to
-.Ql +N .
-If both offsets are omitted,
-.Ql +0+0
-is assumed; if only the first is present, the second defaults to
-.Ql +0 .
-Widths and heights must be between 1 and 10,000, and resolved offsets must be
-between -10,000 and 10,000.
-Neither
-.Ic list\-windows
-nor
-.Ic list\-panes
-currently generates
-.Ql -N
-relative offsets.
-.Pp
-Container cells begin with geometry and hold other layout cells.
-Braces arrange the child cells from left to right and square brackets arrange
-them from top to bottom.
-Pane cells are leaves and begin with a pane ID and z-index, followed by their
-geometry and optional flags.
-Semicolons separate child cells.
-The complete cell forms are:
-.Bd -literal -offset indent
-cell      = container | pane
-container = geometry{cell;cell;...}
-          | geometry[cell;cell;...]
-pane      = %pane-id,z-index:geometry[:flags]
-.Ed
-.Pp
-Pane IDs in the new form must be unique.
-If the layout and target window have the same number of panes and the pane IDs
-exactly match those in the target window, panes are assigned by ID.
-If none match, panes are assigned in tree order, allowing a layout from another
-window to be reused; a partial match is rejected as ambiguous.
-If the target window has fewer panes, cells are removed from the end of the
-layout tree until the counts match, remaining z-indexes are compacted, and
-panes are assigned in tree order.
-A layout cannot be applied to a target window with more panes than it has pane
-cells.
-.Pp
-If a window has only one pane, the root layout cell is itself a pane cell, so
-the pane ID, z-index, geometry and any flags immediately follow the
-.Ql v2:
-prefix.
-The z-index is zero for the front pane and increases towards the back.
-It is present for every pane, including tiled panes.
-Z-indexes must be unique and contiguous from zero, with all floating panes
-before any tiled panes.
-The flags are
-.Ql f
-for floating,
-.Ql h
-for hidden and
-.Ql z
-for zoomed.
-An absent
-.Ql f
-flag means tiled and absent
-.Ql h
-and
-.Ql z
-flags mean visible and unzoomed.
-The
-.Ql h
-and
-.Ql z
-flags cannot be combined.
-.Pp
-For example, this layout contains two tiled panes and one floating pane:
+after the checksum, uses semicolons between cells, and begins pane cells with
+.Ql % .
+For example:
 .Bd -literal -offset indent
 7749,v2:120x40+0+0{%0,1:60x40+0+0;%1,2:59x40+61+0;%2,0:28x8+11+6:f}
 .Ed
 .Pp
-The old form uses commas rather than semicolons to separate child cells
-and does not include z-indexes or flags:
+The old form has no
+.Ql v2:
+prefix, uses commas, and does not include z-indexes or flags.
+For example:
 .Bd -literal -offset indent
-old-geometry = widthxheight,x,y
-cell         = old-geometry{cell,cell,...}
-             | old-geometry[cell,cell,...]
-             | old-geometry[,pane-id]
+89f5,80x24,0,0{39x24,0,0,0,40x24,40,0,1}
 .Ed
 .Pp
-Pane IDs are optional in old layouts and are ignored when assigning panes.
-This permits layouts from versions of
-.Nm
-before pane IDs were added.
-.Pp
-The checksum is optional.
-If present, it is four hexadecimal digits and covers all non-whitespace text
-after its comma, including any version prefix; a layout with an incorrect
-checksum is rejected.
-Whitespace, including newlines, may appear between layout tokens and does not
-change the checksum, so a layout may be indented for readability.
-A checksum may also appear before any nested cell; nested checksums are
-accepted and checked but are not generated by
-.Ic list\-windows .
-Version prefixes are accepted only on the outer layout, not on nested cells.
-A layout must use one form consistently.
-In an old layout, panes are treated as tiled, visible and unzoomed.
 Outside control mode,
 .Ql window_layout
 and