diff --git a/docs/lang/syntax.md b/docs/lang/syntax.md
index 72ec4fe6..41e14c03 100644
--- a/docs/lang/syntax.md
+++ b/docs/lang/syntax.md
@@ -149,6 +149,19 @@ If the declared size and the size deduced from the `<initial_values>` don't matc
 * `<initial_values>` is an array literal, see [Literals](./literals.md).
 Local arrays can have initial values only if they're const.
 
+Each array has an associated constant defined that contains its length
+and, if the indices are numeric, another constant that contains the last index of the array:
+
+    array x[5]
+    x.length        // equals 5
+    x.lastindex     // equals 4
+    
+    enum e { ... }
+    array y[e]
+    y.length        // equals e.count
+    // y.lastindex  // doesn't exist
+    
+
 TODO
 
 ### Function declarations
diff --git a/src/main/scala/millfork/env/Environment.scala b/src/main/scala/millfork/env/Environment.scala
index 13bcd4bc..53674752 100644
--- a/src/main/scala/millfork/env/Environment.scala
+++ b/src/main/scala/millfork/env/Environment.scala
@@ -1458,6 +1458,15 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
                 }
                 if (length < 256) {
                   addThing(ConstantThing(arrayName + ".length", lengthConst, b), stmt.position)
+                } else {
+                  addThing(ConstantThing(arrayName + ".length", lengthConst, w), stmt.position)
+                }
+                if (length > 0 && indexType.isArithmetic) {
+                  if (length <= 256) {
+                    addThing(ConstantThing(arrayName + ".lastindex", NumericConstant(length - 1, 1), b), stmt.position)
+                  } else {
+                    addThing(ConstantThing(arrayName + ".lastindex", NumericConstant(length - 1, 2), w), stmt.position)
+                  }
                 }
               case _ => log.error(s"Array `${stmt.name}` has weird length", stmt.position)
             }
@@ -1531,6 +1540,15 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
         }
         if (length < 256) {
           addThing(ConstantThing(arrayName + ".length", NumericConstant(length, 1), b), stmt.position)
+        } else {
+          addThing(ConstantThing(arrayName + ".length", NumericConstant(length, 2), w), stmt.position)
+        }
+        if (length > 0 && indexType.isArithmetic) {
+          if (length <= 256) {
+            addThing(ConstantThing(arrayName + ".lastindex", NumericConstant(length - 1, 1), b), stmt.position)
+          } else {
+            addThing(ConstantThing(arrayName + ".lastindex", NumericConstant(length - 1, 2), w), stmt.position)
+          }
         }
     }
   }