linux
diff --git a/man/man1/install.1 b/man/man1/install.1
index 816988e..e070ffb 100644
--- a/man/man1/install.1
+++ b/man/man1/install.1
@@ -36,6 +36,11 @@
.PP
.I INSTALL
can safely be repeated to rebuild the system from scratch.
+.PP
+The system uses different threading implementations on Linux 2.6 and
+later kernels than on 2.4 and earlier;
+and on FreeBSD 5 and later kernels than on FreeBSD 4 and earlier.
+Running binaries from one class on another may not work.
.SH FILES
.TP
.B \*9/lib/moveplan9.files
