diff --git a/compiler/mono/src/layout.rs b/compiler/mono/src/layout.rs
index a7c9111e7c..a38b0b1fa6 100644
--- a/compiler/mono/src/layout.rs
+++ b/compiler/mono/src/layout.rs
@@ -276,6 +276,12 @@ pub enum UnionLayout<'a> {
     /// It has more than one other variant, so they need tag IDs (payloads are "wrapped")
     /// e.g. `FingerTree a : [ Empty, Single a, More (Some a) (FingerTree (Tuple a)) (Some a) ]`
     /// see also: https://youtu.be/ip92VMpf_-A?t=164
+    ///
+    /// nullable_id refers to the index of the tag that is represented at runtime as NULL.
+    /// For example, in `FingerTree a : [ Empty, Single a, More (Some a) (FingerTree (Tuple a)) (Some a) ]`,
+    /// the ids would be Empty = 0, More = 1, Single = 2, because that's how those tags are
+    /// ordered alphabetically. Since the Empty tag will be represented at runtime as NULL,
+    /// and since Empty's tag id is 0, here nullable_id would be 0.
     NullableWrapped {
         nullable_id: u16,
         other_tags: &'a [&'a [Layout<'a>]],
@@ -283,6 +289,14 @@ pub enum UnionLayout<'a> {
     /// A recursive tag union with only two variants, where one is empty.
     /// Optimizations: Use null for the empty variant AND don't store a tag ID for the other variant.
     /// e.g. `ConsList a : [ Nil, Cons a (ConsList a) ]`
+    ///
+    /// nullable_id is a bool because it's only ever 0 or 1, but (as with the NullableWrapped
+    /// variant), it reprsents the index of the tag that will be represented at runtime as NULL.
+    ///
+    /// So for example, in `ConsList a : [ Nil, Cons a (ConsList a) ]`, Nil is tag id 1 and
+    /// Cons is tag id 0 because Nil comes alphabetically after Cons. Here, Nil will be
+    /// represented as NULL at runtime, so nullable_id is 1 - which is to say, `true`, because
+    /// `(1 as bool)` is `true`.
     NullableUnwrapped {
         nullable_id: bool,
         other_fields: &'a [Layout<'a>],