Bottom rail

Bottom rail: Bottom horizontal member of a window sash.
Bow window: A composite of 3 or more windows in a radial or bow formation. Typically consists of casement windows both fixed and operating assembled at 10 degree angles from the wall.

Bottom Rail - The lower rail of the bottom sash of a double-hung window.
Box Sill - A type of sill employing a continuous header with the appearance being responsible for the name.

Meeting rail The bottom rail of the upper sash & the upper rail of the lower sash on a double or single-hung window where the lock is mounted which meets when closing. Also known as Meeting Rail & Lock Rail & Check Rail.
Masonry opening ...

Bottom Rail - The horizontal bottom member of a framed door, casement or lower sash
Bottom Tie - A base tie of a pressed metal door frame ...

The horizontal cross pieces are the top rail, bottom rail, and middle or intermediate rails.

Door sill or storm bar - bar fitted to the bottom rail, designed to keep out rain.
Muntin - vertical central part of the door between panels.
Plinth block - squared blocks on which the architrave sits.

Lift: handle or grip installed on the bottom rail of the lower sash of a double-hung window to make it easier to raise or lower the sash.
Light: (also spelled lite) Glazing framed by muntins and/or sash in a window or door.

An entire railing system along the edge of a balcony or stairwell, including the top rail, bottom rail and balusters.
Bargeboard ...

balustrade - an entire railing system including a top rail and its balusters, and often a bottom rail.

Rail - A minor horizontal structural member (e.g. of a balustrade, fence, gate, door or window). It may be top rail, intermediate rail or bottom rail.
Railing - A balustrade.

openings (if they exist) between the bottom rail and treads are typically no bigger than 6 inches (152 mm).
minimum headroom
maximum vertical height between floors or landings. This allows people to rest and limits the height of a fall.

See also: Frame, Door, Rail, Masonry, Member