diff --git a/lib/arch/arch.nim b/lib/arch/arch.nim
index a11bfb21f8..c8ae3da1ca 100644
--- a/lib/arch/arch.nim
+++ b/lib/arch/arch.nim
@@ -6,6 +6,9 @@
 #    See the file "copying.txt", included in this
 #    distribution, for details about the copyright.
 #
+# Architecture-specific optimizations and features.
+# arch.nim can be imported by only a subset of the
+# architectures supported by Nim.
 
 when defined(windows):
   const