Tail gate and Tailboard

Tail gate noun - A gate downstream from a lock or canal that is used to control the flow of water at the lower end.

Tailboard and tail gate are semantically related. In some cases you can use "Tailboard" instead a noun phrase "Tail gate".

Tailboard noun - A gate at the rear of a vehicle; can be lowered for loading.

Tail gate and tailboard are semantically related. Sometimes you can replace phrase "Tail gate" with "Tailboard", this terms are similar.