A  lock  fitting  a  rectangular  shaped  cavity  in  the  edge  of  a  door.  A  round  hole  in  the  face  of  the  door  receives  a  spindle  to  which  knobs  or  levers  are  attached.  If  key  operated,  a  second  round  hole  above  the  first  receives  the  cylinder(s)  and  thumb  turn.  Some  functions  use  two  cylinders  which  is  not  a  violation  of  the  codes  because  the  inside  knob  always  operates.  Some  functions  use  two  cylinders  which  sometimes  is  a  violation  of  codes  because  the  inside  key  projects  a  dead  bolt  or  locks the inside knob which can only  be  unlocked  by  key.  (This  example  of  key  operation  on  the  inside  applies  equally  to  other  types  of  locks  and  is  mentioned  under  mortise  locks  only  because  it  originated  with  them.)