Its best because its flat - Nite Ize S-Biner Dual Carabiner Aluminum GWP

Not the best locking system. The metal on the linking spring is a little weak. The slim size is its best advantage. There are better binders for your keys in my opinion. I used a piece of wire to lock the bottom clip in once I put my keys on it just to lock it down and prevent the clip from opening and dropping my keys.