diff --git a/docs/specification/new-spec/abci.md b/docs/specification/new-spec/abci.md
new file mode 100644
index 00000000..d942c800
--- /dev/null
+++ b/docs/specification/new-spec/abci.md
@@ -0,0 +1,48 @@
+# Application Blockchain Interface (ABCI)
+
+ABCI is the interface between Tendermint (a state-machine replication engine)
+and an application (the actual state machine).
+
+The ABCI message types are defined in a [protobuf
+file](https://github.com/tendermint/abci/blob/master/types/types.proto).
+For full details on the ABCI message types and protocol, see the [ABCI
+specificaiton](https://github.com/tendermint/abci/blob/master/specification.rst).
+For additional details on server implementation, see the [ABCI
+readme](https://github.com/tendermint/abci#implementation).
+
+Here we provide some more details around the use of ABCI by Tendermint and
+clarify common "gotchas".
+
+## Validator Updates
+
+Updates to the Tendermint validator set can be made by returning `Validator`
+objects in the `ResponseBeginBlock`:
+
+```
+message Validator {
+    bytes pub_key = 1;
+    int64 power = 2;
+}
+```
+
+The `pub_key` is the Amino encoded public key for the validator. For details on
+Amino encoded public keys, see the [section of the encoding spec](./encoding.md#public-key-cryptography).
+
+For example, the 32-byte Ed25519 pubkey
+`76852933A4686A721442E931A8415F62F5F1AEDF4910F1F252FB393F74C40C85` would be
+Amino encoded as
+`1624DE622076852933A4686A721442E931A8415F62F5F1AEDF4910F1F252FB393F74C40C85`
+
+The `power` is the new voting power for the validator, with the
+following rules:
+
+- power must be non-negative
+- if power is 0, the validator must already exist, and will be removed from the
+  validator set
+- if power is non-0:
+    - if the validator does not already exist, it will be added to the validator
+      set with the given power
+    - if the validator does already exist, its power will be adjusted to the given power
+
+
+## Query
diff --git a/docs/specification/new-spec/encoding.md b/docs/specification/new-spec/encoding.md
index de747892..83154c85 100644
--- a/docs/specification/new-spec/encoding.md
+++ b/docs/specification/new-spec/encoding.md
@@ -33,7 +33,8 @@ Tendermint uses Amino to distinguish between different types of private keys,
 public keys, and signatures. Additionally, for each public key, Tendermint
 defines an Address function that can be used as a more compact identifier in
 place of the public key. Here we list the concrete types, their names,
-and prefix bytes for public keys and signatures. Note for brevity we don't
+and prefix bytes for public keys and signatures, as well as the address schemes
+for each PubKey. Note for brevity we don't
 include details of the private keys beyond their type and name, as they can be
 derrived the same way as the others using Amino.