B-type Umbrella mount. Fits most standard light stands as well as most speedlights and triggers. The umbrella rod opening fits a standard umbrella shaft and the cold shoe is adjustable from 17-23mm. Made of ABS (acrylonitrile butadiene styrene) plastic, these are lightweight and very sturdy. Also, they are inexpensive enough that if you happen to break one (which is rare) it is easy on the wallet.