At one time, each squadron had its club, and some flying squadrons continue to maintain a bar in the squadron facilities for officer and enlisted aircrew to this day, but most disappeared after World War II and the various Airmen's Clubs, Senior NCO Clubs and Officers' Clubs became facilities of a base rather than a unit. Most are now officially referred to as officer or enlisted clubs; the term "mess" or "officers' open mess" having largely disappeared from the Air Force lexicon. Though a few bases (usually major training bases) have separate Airmen's Clubs for junior enlisted and NCO Clubs for noncommissioned officers, this is no longer normally the case. Physically separate Officers' Clubs still exist at some installations; however, smaller Air Force installations may have one consolidated club with separate lounges. Membership is voluntary, though highly encouraged for senior NCOs and officers. Most NCO and Officers Clubs contain a sit-down restaurant in addition to social lounges, meeting/dining rooms, and bars.