diff --git a/compose/compose-file.md b/compose/compose-file.md
index 40dbfda3b5c5..7024e6f7d37a 100644
--- a/compose/compose-file.md
+++ b/compose/compose-file.md
@@ -1143,8 +1143,20 @@ string. In the example above, if `EXTERNAL_PORT` is not set, the value for the
 port mapping is `:5000` (which is of course an invalid port mapping, and will
 result in an error when attempting to create the container).
 
-Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended shell-style
-features, such as `${VARIABLE-default}` and `${VARIABLE/foo/bar}`, are not
+In the case of environment variables that are not set, it is also possible to
+define default values  using one of the following syntax variants:
+
+
+* To provide a default value that will be used if `VARIABLE` is either *unset* or *empty*:
+
+  `${VARIABLE:-default}`
+
+* To provide a default value only if `VARIABLE` is *empty*:
+
+  `${VARIABLE-default}`
+
+Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended
+shell-style features, such as `${VARIABLE/foo/bar}`, are not
 supported.
 
 You can use a `$$` (double-dollar sign) when your configuration needs a literal