diff --git a/docs/sources/reference/builder.md b/docs/sources/reference/builder.md
index adf42ae764..d837541aa2 100644
--- a/docs/sources/reference/builder.md
+++ b/docs/sources/reference/builder.md
@@ -113,18 +113,30 @@ images.
 > replacement at the time. After 1.3 this behavior will be preserved and
 > canonical.
 
-Environment variables (declared with [the `ENV` statement](#env)) can also be used in
-certain instructions as variables to be interpreted by the `Dockerfile`. Escapes
-are also handled for including variable-like syntax into a statement literally.
+Environment variables (declared with [the `ENV` statement](#env)) can also be
+used in certain instructions as variables to be interpreted by the
+`Dockerfile`. Escapes are also handled for including variable-like syntax
+into a statement literally.
 
 Environment variables are notated in the `Dockerfile` either with
 `$variable_name` or `${variable_name}`. They are treated equivalently and the
 brace syntax is typically used to address issues with variable names with no
 whitespace, like `${foo}_bar`.
 
+The `${variable_name}` syntax also supports a few of the standard `bash`
+modifiers as specified below:
+
+* `${variable:-word}` indicates that if `variable` is set then the result
+  will be that value. If `variable` is not set then `word` will be the result.
+* `${variable:+word}` indiates that if `variable` is set then `word` will be
+  the result, otherwise the result is the empty string.
+
+In all cases, `word` can be any string, including additional environment
+variables.
+
 Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`,
 for example, will translate to `$foo` and `${foo}` literals respectively.
- 
+
 Example (parsed representation is displayed after the `#`):
 
     FROM busybox