What Is a Mortise?

A mortise is a type of joint used to secure two elements of a structure together. Often regarded as a woodworking joint, the mortise has a broader use historically. One of these is its occurrence at Stonehenge, the famous megalith formation in England, in which the so-called sarsens---the large upright stones---are secured to the lintel stones that sit atop them by mortise joints.

The Joint

A mortise serves little or no function without its counterpart, the tenon. The mortise is a hallowed-out portion (usually rectangular) in a piece of wood or stone. It is intended to receive the tenon, the inverse of the mortise. The tenon is a portion that projects from the end of piece of wood or stone. A craftsman produces the tenon by reducing the size at the end of the piece, removing enough material to leave just that part that will insert into the mortise. The tenon is shaped to match the shape of the hollowed mortise. Mortise and tenons are like gloves and fingers.

History

Before modern framing for houses came into wide use in America, the traditional method of building was timber framing or post and beam. And the mortise and tenon joint was the hallmark of this style of building. Settlers brought the craftsmanship from the old country and turned the wealth of available timber into houses and outbuildings. Many of these are in sound structural condition today, even after two or three centuries. The mortise and tenon joint is at the core of the reason for their longevity.

Post and Beam

The post-and-beam builder made tenons at the ends of large horizontal beams and hollowed-out mortises in the upright posts at the building's corners and mid-section. These joints fit together---tenon inside mortise---and were secured with wooden pegs. The builder drove the pegs, called trunnels, through slightly offset holes that pierced the mortise and tenon. Once these trunnels were driven home, the mortise and tenon joint was locked in tightly and was extraordinarily strong and durable.

Furniture

In quality furniture-making the mortise and tenon joint is in wide usage. It is structurally and functionally the same joint as that in traditional post and beam building. The difference is really a matter of size and scale. In quality wooden tables, for example, the apron part of the frame under the tabletop is typically secured to the tops of the legs using pegged mortise and tenon joints.

Modernity

The mortise joint in today's building is far from dead. Post and beam construction has enjoyed something of a resurgence, with some builders now specializing in it. And fine furniture still features the mortise and tenon joint.