diff --git a/src/doc/book/primitive-types.md b/src/doc/book/primitive-types.md
index 69040931de676..2a4b7ba37f25b 100644
--- a/src/doc/book/primitive-types.md
+++ b/src/doc/book/primitive-types.md
@@ -97,9 +97,10 @@ and `i64` is a signed, 64-bit integer.
 
 ## Variable-size types
 
-Rust also provides types whose size depends on the size of a pointer of the
-underlying machine. These types have ‘size’ as the category, and come in signed
-and unsigned varieties. This makes for two types: `isize` and `usize`.
+Rust also provides types whose particular size depends on the underlying machine
+architecture. Their range is sufficient to express the size of any collection, so
+these types have ‘size’ as the category. They come in signed and unsigned varieties
+which account for two types: `isize` and `usize`.
 
 ## Floating-point types