diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index bb8e8c68..f8367cde 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -86,6 +86,7 @@
   "module_use.ml": 3,
   "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
+  "number_words.ml": 106,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
diff --git a/lib/ocaml/baseline/number_words.ml b/lib/ocaml/baseline/number_words.ml
new file mode 100644
index 00000000..467b5eaa
--- /dev/null
+++ b/lib/ocaml/baseline/number_words.ml
@@ -0,0 +1,19 @@
+let number_to_words n =
+  match n with
+  | 1 -> "one" | 2 -> "two" | 3 -> "three" | 4 -> "four" | 5 -> "five"
+  | 6 -> "six" | 7 -> "seven" | 8 -> "eight" | 9 -> "nine"
+  | 10 -> "ten" | 11 -> "eleven" | 12 -> "twelve"
+  | 13 -> "thirteen" | 14 -> "fourteen" | 15 -> "fifteen"
+  | 16 -> "sixteen" | 17 -> "seventeen" | 18 -> "eighteen" | 19 -> "nineteen"
+  | _ -> ""
+
+let total_letters limit =
+  let total = ref 0 in
+  for i = 1 to limit do
+    total := !total + String.length (number_to_words i)
+  done;
+  !total
+
+;;
+
+total_letters 19
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 64e0a2e2..fee3ee91 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-10 Phase 5.1 — number_words.ml baseline (letter count of
+  numbers 1-19 spelled out = 106). 19-arm match dispatch returning
+  the English word for each number. Sums lengths over 1..19. Real
+  PE17 covers 1..1000 (answer 21124) but requires more elaborate
+  number-to-words logic. Tests literal-pattern match with many arms.
+  128 baseline programs total.
 - 2026-05-10 Phase 5.1 — palindrome_sum.ml baseline (sum of 3-digit
   palindromes = 49500). 90 palindromes between 100 and 999 (form
   aba; 9 choices for a, 10 for b). Sum = 49500 = 90 * 550 (mean