The sprockets in a cassette are usually held together by three small bolts or rivets for ease of installation. These keep the sprockets and spacers in the correct order and position when they are removed from the freehub body. When the sprockets need to be replaced due to wear or the user wishes to change gear ratios available, only the sprockets are replaced, not the ratchet mechanism. Cassettes also allow the use of sprockets with fewer teeth, as in micro drive systems.