diff --git a/config.md b/config.md
index 3f6dc035a..b35fcfda9 100644
--- a/config.md
+++ b/config.md
@@ -12,7 +12,7 @@ This specification uses the following terms:
 ### [Layer](layer.md)
 
 Image filesystems are composed of *layers*.
-Each layer represents a set of filesystem changes in a tar-based [layer format](layer.md), recording files to be added, changed, or deleted relative to its parent layer.
+Each layer represents a set of filesystem changes in a compressed tar-based [layer format](layer.md), recording files to be added, changed, or deleted relative to its parent layer.
 Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer.
 Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if they were one cohesive filesystem.