There are two sizes available for this cup holder. It can mount to any vertical rail or stanchion and can be used as a single drink holder, or it folds out for two! Easy to install and remove with just a screwdriver. The available rail sizes are 1″ and 7/8″. Most MacGregor Stanchions and 26M stern rail seats are 1″ the Blue Water Yachts Pedestal Guards will be 7/8″ except for the 26X Cockpit table which is 1″. To be sure you need to measure the rail you want the cup holder to fit! Just put a tape measure behind it and look on both side to line it up, it should be easy to tell which you have.