diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index dea6a18..824efe7 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -81,6 +81,10 @@
 functions can be used to convert between the window/tab number and the
 identifier.  There is also the window number, which may change whenever
 windows are opened or closed, see |winnr()|.
+The window number is only valid in one specific tab.  The window ID is valid
+across tabs.  For most functions that take a window ID or a window number, the
+window number only applies to the current tab, while the window ID can refer
+to a window in any tab.
 
 Each buffer has a unique number and the number will not change within a Vim
 session.  The |bufnr()| and |bufname()| functions can be used to convert