display-planes on tty

diff --git a/lispref/frames.texi b/lispref/frames.texi
index 2692f61d7f8fff41b949a6acb608fbc0a8697f67..17b6794129029eda3c2143d895a253ea23d78651 100644 (file)
--- a/lispref/frames.texi
+++ b/lispref/frames.texi
@@ -1699,6 +1699,7 @@ quickly.
 @tindex display-planes
 This function returns the number of planes the display supports.
 This is typically the number of bits per pixel.
+For a tty display, it is log to base two of the number of colours supported.
 @end defun
 
 @defun display-visual-class &optional display