diff --git a/org/box-renderable.org b/org/box-renderable.org
index 6fbf00e..d16bca0 100644
--- a/org/box-renderable.org
+++ b/org/box-renderable.org
@@ -335,6 +335,24 @@ objects and they are accessible via ~text-spans~.
     (is (= (length (text-spans tx)) 2))
     (is (string= (span-text (elt (text-spans tx) 0)) "Hello"))
     (is-true (span-bold (elt (text-spans tx) 0)))))
+
+(test test-char-width-ascii
+  "ASCII characters (< 128) have width 1."
+  (is (= 1 (char-width #\a)))
+  (is (= 1 (char-width #\Space)))
+  (is (= 1 (char-width #\@))))
+
+(test test-char-width-tab
+  "Tab character has width 8."
+  (is (= 8 (char-width #\Tab))))
+
+(test test-char-width-cjk
+  "CJK characters have width 2."
+  (is (= 2 (char-width #\日))))
+
+(test test-char-width-null
+  "Null character has width 0."
+  (is (= 0 (char-width #\Nul))))
 #+END_SRC
 
 * Implementation