There are a few different ones so I'll hit off on the most commonly used. The flat tops are the most commonly used. They have a straight hard line that separates the distance from the reading portion. Flat top 28 - Bifocal width will be 28mm Flat top 35 - Bifocal width will be 35mm. There are also rounded bifocal - The bifocal is round instead of the traditional half circle. There is another style called blended bifocal which is less noticeable because it does not stand out as the lined bifocal by eliminating the hard line and introducing a softer, smoother and rounded edge to the reading section.Then we have executive style which is essentially a flat top but with altered measurements that make the reading portion the entirety of the Rx below the line and distance Rx above it. Lastly we have Double D - Theses lenses have a bifocal on top of the lens and on the bottom