A gear case, also known as a chain case, is an enclosure for the bicycle chain and sprocket assemblages commonly employed by utility bicycles. It serves to protect the chain from mud and dirt and tends to fully enclose the drive train. It may also contain an oil bath to keep the chain lubricated. Modern examples are usually moulded in plastic. Similar devices may be found in connection with chains used on larger vehicles and machinery.
There are also a variety of smaller structures called chainguards which, rather than protecting the chain, serve only to protect the rider's clothes from being soiled or trapped in the chain rings.