A crown is also known as a cap. It sits on the tooth as a protective cover to prevent a tooth from chipping or breaking when chewing. A crown extends the life of the tooth. Signs for the need of a crown are large fillings, a tooth with a crack in it, teeth that are broken or have large chips where a filling would not hold, and teeth that have had root canals. Crowns can last 10-15-20 years or longer if properly done and maintained.