diff --git a/css-grid-1/Overview.bs b/css-grid-1/Overview.bs
index a2c52a2d5b2..739ae20b40b 100644
--- a/css-grid-1/Overview.bs
+++ b/css-grid-1/Overview.bs
@@ -3276,6 +3276,9 @@ With a Grid Container as Containing Block
it is instead treated as specifying ''auto''
(instead of creating new implicit grid lines).
+ Note: Remember that implicit lines are assumed to have all line names,
+ so a referenced line might exist even though it is not explicitly named.
+
If the placement only contains a grid span,
replace it with the two ''auto'' lines in that axis.
(This happens when both grid-placement properties in an axis contributed a span originally,