Avid disc brake adapters allow Avid disc brake calipers to mount on a variety of frames and forks. IS Mount: Refers to 51mm International Standard type fork or frame brake mounts. This adapter mounts to the frame or fork with two side-facing bolts. IS mounts are common on older style Fox, RockShox and Marzocchi forks, and are still the standard for most frame manufacturers. Post Mount: Refers to post mount type fork or frame brake mounts. Using this style the adapter mounting bolts will thread straight into the "posts" on lower fork leg. Most new suspension forks will use the post style brake mounts. Very few frames will use post style mounting for rear brake at this time.