Bay: a division of space that is repeated within a building, a three bay house would have three spaces repeated along one side as in two rooms and a hall.

  • A space that projects from the rest of the building as in a bay window.
  • Architecture:
    • any of a number of similar major vertical divisions of a large interior, wall, etc.: The nave is divided into six bays.
    • a division of a window between a mullion and an adjoining mullion or jamb.
    • bay window
    • an alcove or recess in a wall

Mullion: a vertical structural element which divides adjacent window units. When used to support glazing, they are teamed with horizontal supporting elements called “transoms“.

  • In the commercial door industry, the term is also applied to a piece of hardware that divides the opening of a pair of doors.
  • A mullion acts as a structural member.  It carries the dead load of the weight above the opening and/or the wind load acting on the window unit back to the building structure. The term is also properly applied to very large and deep structural members in many curtain wall systems.

Muntin or Muntin bar:  is sometimes similar in appearance to a mullion, but they are not structural.  A muntin is a strip of wood or metal separating and holding panes of glass in a window.  Muntins divide a single window into a grid system of smaller panes of glass, called “lights” or “lites”.

  • Muntins can be found in doors and windows.
  • Windows with “true divided lights” make use of thin muntins positioned between individual panes of glass.  True divided lights are not typically seen in modern windows as they decrease performance and are more expensive/difficult to implement.
  • In contemporary windows muntins frequently do not actually separate panes of glass, but are decorative elements attached to the inside and/or outside of panes of glass to give the appearance of multiple smaller panes.  This is commonly called a “simulated divided light.”
  • Muntins are frequently mistakenly referred to as mullions.

Jamb:  One of a pair of vertical posts or pieces that together form the sides of a door, window frame, or fireplace, for example.

  • Architecture, Building Trades:
    • either of the vertical sides of a doorway, arch, window, or other opening.
    • either of two stones, timbers, etc., forming the sidepieces for the frame of an opening.
    • A projecting mass or columnar part.

Transom: a bar of wood or stone across the top of a door or window

  • a crosspiece separating a door or the like from a window or fanlight above it.
  • Also called transom light, transom window.
  • a window above such a crosspiece.
  • a crossbar of wood or stone, dividing a window horizontally
  • Compare mullion.  Also called: traverse

Fanlight:  A fanlight is a window, semicircular or semi-elliptical in shape, with glazing bars or tracery sets radiating out like an open fan, It is placed over another window or a doorway and is sometimes hinged to a transom. The bars in the fixed glazed window spread out in the manner a sunburst. It is also called a “sunburst light”.

  • A chiefly British use of the word is to mean “transom“.

Traverse:  a transverse gallery or loft of communication in a church or other large building.

  • Something being or lying across, such as a transom
  • A gallery or loft inside a building that crosses it
  • construction: Crossbeam: something that is set across a gap or lies crosswise, e.g. a structural member of a building
  • buildings :  Gallery: a gallery or loft that crosses from side to side inside a building
  • buildings :  Barrier within building: a railing, curtain, screen, or partition forming a barrier within a building
  • civil engineering:  Survey using intersecting straight lines: a survey made using a series of intersecting straight lines of known length whose angles of intersection are measured for recording on a map or in a table of data

Tracery:  window ornamentation: decorative ribs in windows, especially medieval church windows, and screens

  • interlaced pattern: a decorative pattern of interlaced lines, especially one that resembles the form or patterns found in church windows
  • In architecture, Tracery is the stonework elements that support the glass in a Gothic window. The term probably derives from the ‘tracing floors’ on which the complex patterns of late Gothic windows were laid out