The Black Diamond Gridlock Screwgate is a locking carabiner designed specifically for belaying.
The design uses an isolation gate feature to give the biner a small section at the bottom for the belay loop to sit, and this prevents the carabiner from rotating, keeping the belay device in the correct position. The Gridlock's I-beam spine transitions into a large rope bearing surface meaning that most belay devices will work with it, and the larger radius is a little kinder to your climbing rope when belaying and abseiling.
No longer is the DMM Belay Master the main contender for a carabiner to prevent cross-loading when belaying!
Note: Cross-loading is when the carabiner rotates when belaying, and the belay loop and belay device align themselves opposite the short or gated sides, rather than down the long length of the carabiner. Carabiners are substantially weaker in this alignment so it is best avoided.