diff --git a/bip-0138.md b/bip-0138.md
index a999f92e..06f84d0f 100644
--- a/bip-0138.md
+++ b/bip-0138.md
@@ -369,7 +369,16 @@ remaining bytes are `PADDING`. A `PAYLOAD` MUST contain at least one item (see
 
 `LENGTH`: variable-length integer representing the length of `DATA` in bytes.
 
-For all `TYPE` values except `0x01`, `LENGTH` MUST be present.
+For all `TYPE` values except `0x01`, `LENGTH` MUST be present.[^length-required]
+
+[^length-required]: **Why is `LENGTH` mandatory?**
+    `LENGTH` is what lets a parser skip a content type it does not implement and still
+    read the items that follow; without it, the parser cannot know where `DATA` ends.
+    A string note placed before a descriptor, for example, must not stop an
+    implementation that only handles descriptors from recovering that descriptor. A
+    type whose `DATA` is always empty still carries a zero `LENGTH` for this reason.
+    `0x01` is exempt because every implementation handles it, so it is never skipped
+    blindly.
 
 `DATA`: variable-length field whose encoding depends on `TYPE`.