diff --git a/src/basic-syntax/compound-types.md b/src/basic-syntax/compound-types.md
index 7cb2faf2..5a2cfc76 100644
--- a/src/basic-syntax/compound-types.md
+++ b/src/basic-syntax/compound-types.md
@@ -26,16 +26,21 @@ fn main() {
 ```
 
 <details>
-    
+
 Key points:
-    
+
 Arrays:
-    
+
 * Arrays have elements of the same type, `T`, and length, `N`, which is a compile-time constant.
+  Note that the length of the array is *part of its type*, which means that `[u8; 3]` and
+  `[u8; 4]` are considered two different types.
 
 * We can use literals to assign values to arrays.
 
-* In the main function, the print statement asks for the debug implementation with the `?` format parameter: `{a}` gives the default output, `{a:?}` gives the debug output.
+* In the main function, the print statement asks for the debug implementation with the `?` format
+  parameter: `{}` gives the default output, `{:?}` gives the debug output. We
+  could also have used `{a}` and `{a:?}` without specifying the value after the
+  format string.
 
 * Adding `#`, eg `{a:#?}`, invokes a "pretty printing" format, which can be easier to read.
 
@@ -45,6 +50,11 @@ Tuples:
 
 * Tuples group together values of different types into a compound type.
 
-* Fields that can be accessed by the period and the index of the value, e.g. `t.0`, `t.1`.
+* Fields of a tuple can be accessed by the period and the index of the value, e.g. `t.0`, `t.1`.
+
+* The empty tuple `()` is also known as the "unit type". It is both a type, and
+  the only valid value of that type - that is to say both the type and its value
+  are expressed as `()`. It is used to indicate, for example, that a function or
+  expression has no return value, as we'll see in a future slide.
 
 </details>