Cover collecting is the generic term for collecting stamps on envelopes. It includes two broad categories: Postal historical covers, that is, stamps on envelopes collected to show the stamps as they were used for postal purposes, and Philatelic covers, that is, covers made for collectors, usually to commemorate some special event. Philatelic covers come in four broad types: First Day Covers, Naval or Ship covers, Airmail related covers, and Space covers. In most cases, philatelic covers grew out of a small group of classic preparers or "accidentals"