diff --git a/source/runtime.c b/source/runtime.c
index da82e79f..903d7518 100644
--- a/source/runtime.c
+++ b/source/runtime.c
@@ -11443,6 +11443,15 @@ static JSValue js_cell_is_text (JSContext *ctx, JSValue this_val, int argc, JSVa
   return JS_NewBool (ctx, JS_IsText (argv[0]));
 }
 
+static JSValue js_cell_is_letter (JSContext *ctx, JSValue this_val, int argc, JSValue *argv) {
+  if (argc < 1) return JS_FALSE;
+  JSValue val = argv[0];
+  if (!JS_IsText (val)) return JS_FALSE;
+  if (js_string_value_len (val) != 1) return JS_FALSE;
+  uint32_t c = js_string_value_get (val, 0);
+  return JS_NewBool (ctx, (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z'));
+}
+
 /* is_proto(val, master) - check if val has master in prototype chain */
 static JSValue js_cell_is_proto (JSContext *ctx, JSValue this_val, int argc, JSValue *argv) {
   if (argc < 2) return JS_FALSE;
@@ -11635,6 +11644,7 @@ static void JS_AddIntrinsicBaseObjects (JSContext *ctx) {
     js_set_global_cfunc(ctx, "is_stone", js_cell_is_stone, 1);
     js_set_global_cfunc(ctx, "is_text", js_cell_is_text, 1);
     js_set_global_cfunc(ctx, "is_proto", js_cell_is_proto, 2);
+    js_set_global_cfunc(ctx, "is_letter", js_cell_is_letter, 1);
 
     /* Utility functions */
     js_set_global_cfunc(ctx, "apply", js_cell_fn_apply, 2);