▁plane ▁parallel ▁to ▁the ▁frame , ▁by ▁fol ding ▁in ▁angles ▁on ▁a ▁parallel ▁plane , ▁or ▁by ▁spin ning ▁along ▁an ▁axis ▁at ▁the ▁center ▁of ▁the ▁frame ) ▁to ▁allow ▁or ▁prevent ▁ing ress ▁or ▁e gress . ▁In ▁most ▁cases , ▁a ▁door ' s ▁interior ▁matches ▁its ▁exterior ▁side . ▁But ▁in ▁other ▁cases ▁( e . g ., ▁a ▁vehicle ▁door ) ▁the ▁two ▁sides ▁are ▁rad ically ▁different . ▁ ▁Do ors ▁often ▁incorpor ate ▁lock ing ▁mechan isms ▁to ▁ensure ▁that ▁only ▁some ▁people ▁can ▁open ▁them . ▁Do ors ▁can ▁have ▁devices ▁such ▁as ▁knock ers ▁or ▁door b ells ▁by ▁which ▁people ▁outside ▁can ▁announ ce ▁their ▁presence ▁and ▁sum mon ▁someone ▁either ▁to ▁open ▁the ▁door ▁for ▁them ▁or ▁give ▁permission ▁to ▁open ▁and ▁enter . ▁A part ▁from ▁providing ▁access ▁into ▁and ▁out ▁of ▁a ▁space , ▁doors ▁can ▁have ▁the ▁secondary ▁functions ▁of ▁ens uring ▁priv acy ▁by ▁prevent ing ▁unw anted ▁attention ▁from ▁out s iders , ▁of ▁separ ating ▁areas ▁with ▁different ▁functions , ▁of ▁allowing ▁light ▁to ▁pass ▁into ▁and ▁out ▁of ▁a ▁space , ▁of ▁cont rolling ▁vent ilation ▁or ▁air ▁draft s ▁so ▁that ▁inter iors ▁may ▁be ▁more ▁effectively ▁he ated ▁or ▁co o led , ▁of ▁d amp ening ▁noise , ▁and ▁of ▁blocking ▁the ▁spread ▁of ▁fire . ▁ ▁Do ors ▁may ▁have ▁a est h etic , ▁symbol ic , ▁rit ual istic ▁purposes . ▁To ▁be ▁given ▁the ▁key ▁to ▁a