▁of ▁the ▁clos et ▁at ▁a ▁time . ▁The ▁doors ▁in ▁a ▁by pass ▁unit ▁will ▁overlap ▁slightly ▁when ▁viewed ▁from ▁the ▁front , ▁in ▁order ▁not ▁to ▁have ▁a ▁visible ▁gap ▁between ▁them . ▁ ▁Do ors ▁which ▁slide ▁inside ▁a ▁wall ▁cav ity ▁are ▁called ▁pocket ▁doors . ▁This ▁type ▁of ▁door ▁is ▁used ▁in ▁tight ▁spaces ▁where ▁priv acy ▁is ▁also ▁required . ▁The ▁door ▁sl ab ▁is ▁mounted ▁to ▁roll er ▁and ▁a ▁track ▁at ▁the ▁top ▁of ▁the ▁door ▁and ▁sl ides ▁inside ▁a ▁wall . ▁ ▁Sl iding ▁glass ▁doors ▁are ▁common ▁in ▁many ▁houses , ▁particularly ▁as ▁an ▁entrance ▁to ▁the ▁back yard . ▁Such ▁doors ▁are ▁also ▁popular ▁for ▁use ▁for ▁the ▁ent ran ces ▁to ▁commercial ▁structures , ▁although ▁they ▁are ▁not ▁counted ▁as ▁fire ▁exit ▁doors . ▁The ▁door ▁that ▁moves ▁is ▁called ▁the ▁" active ▁leaf ", ▁while ▁the ▁door ▁that ▁remains ▁fixed ▁is ▁called ▁the ▁" in active ▁leaf ". ▁ ▁Rot ating ▁doors ▁ ▁A ▁revol ving ▁door ▁has ▁several ▁wings ▁or ▁leaves , ▁generally ▁four , ▁radi ating ▁from ▁a ▁central ▁sh aft , ▁forming ▁compart ments ▁that ▁rotate ▁about ▁a ▁vertical ▁axis . ▁A ▁revol ving ▁door ▁allows ▁people ▁to ▁pass ▁in ▁both ▁directions ▁without ▁coll iding , ▁and ▁forms ▁an ▁air lock ▁maintain ing ▁a ▁se al ▁between ▁inside ▁and ▁out . ▁ ▁A ▁pivot ▁door , ▁instead ▁of ▁h ing es , ▁is ▁supported ▁on ▁a ▁bearing ▁some ▁distance ▁away ▁from ▁the ▁edge , ▁so ▁that