diff --git a/src/x/__tests__/xIsMonotonic.test.ts b/src/x/__tests__/xIsMonotonic.test.ts
index f0e6ed91..a1071fe8 100644
--- a/src/x/__tests__/xIsMonotonic.test.ts
+++ b/src/x/__tests__/xIsMonotonic.test.ts
@@ -55,3 +55,14 @@ test('xIsMonotonic constant series', () => {
 
   expect(xIsMonotonic(array)).toBe(1);
 });
+
+test('xIsMonotonic reads the direction of a pair', () => {
+  expect(xIsMonotonic([3, 5])).toBe(1);
+  expect(xIsMonotonic([5, 3])).toBe(-1);
+  expect(xIsMonotonic([5, 5])).toBe(1);
+});
+
+test('xIsMonotonic on fewer than two values', () => {
+  expect(xIsMonotonic([])).toBe(1);
+  expect(xIsMonotonic([5])).toBe(1);
+});
diff --git a/src/x/xIsMonotonic.ts b/src/x/xIsMonotonic.ts
index 6e4512f1..f846a01f 100644
--- a/src/x/xIsMonotonic.ts
+++ b/src/x/xIsMonotonic.ts
@@ -6,7 +6,7 @@ import type { NumberArray } from 'cheminfo-types';
  * @returns 1 if monotonic increasing, -1 if monotonic decreasing, 0 if not monotonic.
  */
 export function xIsMonotonic(array: NumberArray): -1 | 0 | 1 {
-  if (array.length <= 2) {
+  if (array.length < 2) {
     return 1;
   }
   if (array[0] === array[1]) {