There are 2 sizes of clamps / SCS clamps, 31.8mm and 34.9mm. 31.8mm - This is for most standard Chromoly and Steel Bars on the market. There is also 34.9mm sizing - This is used for all Aluminium bars, all HIC bars are also 34.9mm in sizing. All bars using a HIC or ICS will require a slit to allow the bars to tighten on the fork. All bars using an SCS Clamp will need to have the slit removed (so it can allow the clamp to tighten correctly)

There are also a wide variety of heights and colours. As a general rule, the taller the clamp, the less likely it is to have your fork or bar snapping as there is more reinforcement. However, quality can also come into play here, certain clamps like a Tilt SCS for example, can be so short due to the quality being so good.