diff --git a/docs/utilities/spacing.md b/docs/utilities/spacing.md
index 53a6ab538a..91dab4f02d 100644
--- a/docs/utilities/spacing.md
+++ b/docs/utilities/spacing.md
@@ -62,7 +62,7 @@ Here are some representative examples of these classes:
 {% endhighlight %}
 
 ### Horizontal centering
-Additionally, Bootstrap also includes an `.mx-auto` class for horizontally centering fixed-width block level content by setting the horizontal margins to `auto`.
+Additionally, Bootstrap also includes an `.mx-auto` class for horizontally centering fixed-width block level content—that is, content that has `display: block` and a `width` set—by setting the horizontal margins to `auto`.
 
 <div class="bd-example">
   <div class="mx-auto" style="width: 200px; background-color: rgba(86,61,124,.15);">