There are a few pseudo standards for "half width" 1U devices, one of the more notable vendors is mikrotik which makes devices that can be mounted as two units in 1U.
I wish there was some kind of firmly defined standard for exactly half of a 1U width so that different manufacturers' devices could be attached together.
The 10" rack is the de facto standard half-width rack. It uses the same spacing as 19" inch racks. My understanding is that two 8.75" devices fit in 19" rack with mounting bracket in the center.
