IF YOUR RAIL MEASURES 1 1/16" YOU HAVE A 1" RAIL. IF YOUR RAIL MEASURES 1 1/4" OR MORE YOU HAVE A 1 1/4" RAIL. PLEASE LOOK AT THE LAST 2 PICTURES TO DETERMINE YOUR RAIL SIZE. Please note: also available in other colors and cup sizes. Check sellers other items for more options. Mounts on any 1 1/4 inch pontoon rail or square tubing. Great for newer pontoon boats with the larger rails or any place you need to hold a drink. It’s strong too. Made with highly polished 3003 aluminum diamond plate so it wont rust, could last you forever. To mount this just remove cup, slide over railing, reinstall the plastic cups, thats it. Best part is, it can still slide on the rail but cant fall off. So you can put it anywhere on the boat. If you go swimming you can turn it so you can hold your drink outside the boat too. DOES NOT HOLD A LARGE CUP OR CAN WITH LARGE COZI