A lancet window is a tall narrow window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

 with a pointed arch

Arch

An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...

 at its top. It acquired the "lancet" name from its resemblance to a lance

Lance

A Lance is a pole weapon or spear designed to be used by a mounted warrior. The lance is longer, stout and heavier than an infantry spear, and unsuited for throwing, or for rapid thrusting. Lances did not have tips designed to intentionally break off or bend, unlike many throwing weapons of the...

. Instances of this architectural motif

Motif (art)

In art, a motif is an element of a pattern, an image or part of one, or a theme. A motif may be repeated in a design or composition, often many times, or may just occur once in a work. A motif may be an element in the iconography of a particular subject or type of subject that is seen in other...

 are most often found in Gothic

Gothic architecture

Gothic architecture is a style of architecture that flourished during the high and late medieval period. It evolved from Romanesque architecture and was succeeded by Renaissance architecture....

 and ecclesiastical structures, where they are often placed singly or in pairs.

The motif first appeared in the French early-Gothic period (c. 1140-1200) and later in the English period of Gothic architecture (1200-1275). So common was the lancet window feature that this era is sometimes known as the "Lancet Period". Strictly speaking, the lancet window should be austere and without tracery

Tracery

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.-Plate tracery:...

. However, it is not uncommon to find the form sub-divided and filled with stained glass, especially when dating from the Gothic-revival period of the nineteenth century.