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diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index e9210b1776..698687b91f 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -370,6 +370,7 @@ The above will run a monolithic (single-process) Synapse with SQLite as the data
     See the [worker documentation](../workers.md) for additional information on workers.
 - Passing `ASYNCIO_REACTOR=1` as an environment variable to use the Twisted asyncio reactor instead of the default one.
 - Passing `PODMAN=1` will use the [podman](https://podman.io/) container runtime, instead of docker.
+- Passing `UNIX_SOCKETS=1` will utilise Unix socket functionality for Synapse, Redis, and Postgres(when applicable).
 
 To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`, e.g:
 ```sh