diff --git a/specs/simple-serialize.md b/specs/simple-serialize.md
index 119022248..ecef6ddfc 100644
--- a/specs/simple-serialize.md
+++ b/specs/simple-serialize.md
@@ -217,6 +217,12 @@ We now define Merkleization `hash_tree_root(value)` of an object `value` recursi
 
 Let `value` be a self-signed container object. The convention is that the signature (e.g. a `"bytes96"` BLS12-381 signature) be the last field of `value`. Further, the signed message for `value` is `signing_root(value) = hash_tree_root(truncate_last(value))` where `truncate_last` truncates the last element of `value`.
 
+## Summaries and expansions
+
+Let `A` be an object derived from another object `B` by replacing some of the (possibly nested) values of `B` by their `hash_tree_root`. We say `A` is a "summary" of `B`, and that `B` is an "expansion" of `A`. Notice `hash_tree_root(A) == hash_tree_root(B)`.
+
+We similarly define "summary types" and "expansion types". For example, [`BeaconBlock`](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#beaconblock) is an expansion type of [`BeaconBlockHeader`](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#beaconblockheader). Notice that objects expand to at most one object of a given expansion type. For example, `BeaconBlockHeader` objects uniquely expand to `BeaconBlock` objects.
+
 ## Implementations
 
 | Language | Project | Maintainer | Implementation |