diff --git a/chapter2.lisp b/chapter2.lisp
index 6a76738..f2bc972 100644
--- a/chapter2.lisp
+++ b/chapter2.lisp
@@ -90,3 +90,10 @@
   (make-interval (- (lower-bound b) (lower-bound a))
 		 (- (upper-bound b) (upper-bound a))))
 
+;; Exercise 2.9
+(defun width-interval (x)
+  "Calculate width of an interval as half of the distance between lower
+and upper bound."
+  (/ (- (upper-bound x) (lower-bound x))
+     2.0))
+