diff --git a/src/hb-ot-tag.c b/src/hb-ot-tag.c
index 4017c88fd..8f62f8499 100644
--- a/src/hb-ot-tag.c
+++ b/src/hb-ot-tag.c
@@ -639,10 +639,8 @@ hb_ot_tag_from_language (hb_language_t language)
     char tag[4];
     int i;
     lang_str += 6;
-#define IS_LETTER(c) (((c) >= 'a' && (c) <= 'z') || ((c) >= 'A' && (c) <= 'Z'))
-#define TO_UPPER(c) (((c) >= 'a' && (c) <= 'z') ? (c) + 'A' - 'a' : (c))
-    for (i = 0; i < 4 && IS_LETTER (lang_str[i]); i++)
-      tag[i] = TO_UPPER (lang_str[i]);
+    for (i = 0; i < 4 && ISALPHA (lang_str[i]); i++)
+      tag[i] = TOUPPER (lang_str[i]);
     for (; i < 4; i++)
       tag[i] = ' ';
     return HB_TAG_CHAR4 (tag);
diff --git a/src/hb-private.h b/src/hb-private.h
index 98dc0bb97..481015cbe 100644
--- a/src/hb-private.h
+++ b/src/hb-private.h
@@ -263,6 +263,13 @@ typedef int hb_mutex_t;
 #define hb_be_uint32_cmp(a,b)	(a[0] == b[0] && a[1] == b[1] && a[2] == b[2] && a[3] == b[3])
 
 
+/* ASCII character handling */
+
+#define ISALPHA(c) (((c) >= 'a' && (c) <= 'z') || ((c) >= 'A' && (c) <= 'Z'))
+#define TOUPPER(c) (((c) >= 'a' && (c) <= 'z') ? (c) - 'a' + 'A' : (c))
+#define TOLOWER(c) (((c) >= 'A' && (c) <= 'Z') ? (c) - 'A' + 'a' : (c))
+
+
 /* Debug */
 
 #ifndef HB_DEBUG