diff --git a/docs/handbook/concepts.rst b/docs/handbook/concepts.rst
index b2611c11b..055821a15 100644
--- a/docs/handbook/concepts.rst
+++ b/docs/handbook/concepts.rst
@@ -24,8 +24,10 @@ To get the number and names of bands in an image, use the
 Modes
 -----
 
-The ``mode`` of an image defines the type and depth of a pixel in the
-image. The current release supports the following standard modes:
+The ``mode`` of an image defines the type and depth of a pixel in the image.
+Each pixel uses the full range of the bit depth. So a 1-bit pixel has a range
+of 0-1, an 8-bit pixel has a range of 0-255 and so on. The current release
+supports the following standard modes:
 
     * ``1`` (1-bit pixels, black and white, stored with one pixel per byte)
     * ``L`` (8-bit pixels, black and white)