Mortise

What Is a Mortise?

A mortise is a purposeful hollow or slot cut into a workpiece to receive another component. In carpentry and joinery, the most common pairing is the mortise-and-tenon joint: the mortise (the hole) accepts the tenon (the tongue) from a second piece, creating a strong mechanical connection. In hardware installation, a mortise lets hinges, lock bodies, strike plates, and other fittings sit flush with the surface so doors and furniture operate smoothly and look clean.

Mortises show up across many trades. Woodworkers use them for chairs, tables, and frames; door installers cut them for hinges and locksets; and even masons and metalworkers create mortises to accept anchors, straps, or mating parts. While the concept is simple, clean mortises demand careful layout, sharp tools, and the right technique.

Where You’ll Use Mortises in DIY and Home Projects

• Hanging interior and exterior doors: hinge leaf mortises in the door and jamb, plus the shallow faceplate mortise for the latch or bolt.
• Installing a mortise lock: a deep pocket in the door edge for the lock body and shallow recesses for the faceplate and strike.
• Building furniture: legs and rails joined with mortise-and-tenon for tables, chairs, beds, and cabinets.
• Window sash and frames: traditional joinery uses narrow, deep mortises.
• Decks and outdoor structures: pegged mortise-and-tenon in timber framing or decorative work.

Common Types and Variations

• Through mortise: goes completely through the workpiece; the tenon is visible on the far side. Strong and decorative on arts-and-crafts furniture.
• Blind (or stopped) mortise: stops before breaking through; hides the tenon end for a cleaner look.
• Stub mortise: shallow mortise that accepts a short tenon, often in light frames or where space is limited.
• Hinge mortise: a shallow recess matching the hinge leaf thickness so the hinge sits flush.
• Lock mortise: a deep cavity in the door edge for the lock body, plus matching shallow mortises for the faceplate and strike.
• Square-ended vs. round-ended: chisel-cut mortises have square ends; router-cut mortises have rounded ends unless squared up with a chisel.

Tools and Methods for Cutting Mortises

• Chisels: A sharp bench chisel can cut hinge and small joinery mortises; a stout mortise chisel excels at deep, narrow joinery.
• Drill press or hand drill: Remove most waste with a brad-point or Forstner bit, then clean up with chisels.
• Router: With an edge guide or template, a router makes quick, accurate mortises; use an upcut spiral bit for clean walls.
• Hollow-chisel mortiser: A stationary machine for square-sided mortises in repeat work.
• Marking tools: Use a marking gauge or mortise gauge to scribe parallel lines, plus a sharp knife and square for crisp outlines.

Quick How-To: Hinge Mortise

1. Position the hinge and trace around it with a knife for a crisp boundary.
2. Set the depth to match hinge leaf thickness; score the perimeter with the knife.
3. Pare or route within the lines to depth. If routing, use a laminate trimmer with a hinge template.
4. Test-fit. The leaf should be flush, not proud or sunken. Square the corners with a chisel if needed.

Quick How-To: Joinery Mortise

1. Mark the mortise location on the “face” and “edge” reference sides. Set mortise width with a gauge (rule of thirds: about one-third of stock thickness).
2. Remove most waste by drilling a row of overlapping holes slightly shy of full depth.
3. Square and refine the walls with a chisel, working from both ends toward the middle to avoid blowing out fibers.
4. Test-fit the tenon; tweak with light paring or a shoulder plane for a snug, hand-pressed fit.

Sizing Guidelines

• Width: About one-third of the workpiece thickness is a good starting point. For 3/4-inch stock, a 1/4-inch mortise is typical.
• Depth/length: Give the tenon enough length for strength—often 1 to 1-1/2 inches in furniture rails. Avoid placing the mortise so close to an edge that it weakens the part.
• Hardware mortises: Match the depth precisely to hardware thickness so parts sit flush and operate without binding.

Tips for Better Results

• Scribe, don’t pencil: Knife lines reduce tear-out and give a positive edge for the chisel.
• Work off reference faces: Mark and cut all joinery from the same sides to keep parts aligned.
• Clamp and back up: Clamp the piece securely and back the cut with scrap to prevent blowout at the ends.
• Keep tools sharp: Hone chisels often; a stropped edge pares cleanly and safely.
• Let the router do the work: Use multiple shallow passes rather than one deep pass to keep the bit cool and the cut accurate.
• Sneak up on depth: For hinge and faceplate mortises, start shallow and test-fit; it’s easier to remove more than to fill back in.

Common Mistakes to Avoid

• Over-widening by prying: Don’t lever a chisel sideways against the walls; chop and pare instead.
• Cutting past layout lines: Stop shy, then pare to the knife line for crisp edges.
• Misaligned pairs: Mortises for opposing parts that don’t line up lead to racking or binding. Use story sticks and consistent references.
• Rounding or squaring mismatch: Router-cut mortises have rounded ends. Either chisel them square or round the tenon/hinge corners to match.
• Too deep or too shallow hardware mortises: Hinges set too deep cause door binding; too shallow leaves the leaf proud and gaps at the knuckle.
• Wrong lock backset: For lock mortises, confirm backset and case size from the template before cutting.
• Skipping pilot holes: For hinges and strikes, drill proper pilot holes to avoid splitting and stripped screws.

Maintenance and Care

• Chisels: Keep backs flat and edges honed; a quick strop during work maintains a keen edge.
• Router bits: Use sharp upcut spirals for clean mortises; retire dull bits that burn or chatter.
• Mortiser tools: Wax the hollow chisel and keep the auger properly set to reduce heat and clogging.
• Finished mortises: Check periodically for hardware loosening; snug screws or add longer ones into framing if needed.

Related Terms

• Tenon, mortise-and-tenon joint, shoulder, cheek, haunch, drawbore pin
• Dado, groove, rabbet (other recesses and slots in woodworking)
• Backset, strike plate, latch bolt (lock terminology)

Practical Examples

• Hang a door: Mark hinge locations from the top of the door and jamb (commonly 7 in. from the top, 11 in. from the bottom, and one centered), scribe the leaf outline, and route or chisel to leaf thickness. Pilot-drill and install screws.
• Build a table: Cut blind mortises in the legs and matching tenons on the aprons, sized to about one-third of the apron thickness. Dry-fit, then glue and clamp; check the assembly for square.
• Install a mortise lock: Use the manufacturer template to mark the lock body on the door edge. Drill overlapping holes to depth, clean up with chisels, then rout shallow faceplate and strike mortises. Test-fit before drilling spindle and keyholes.
• Repair a loose chair joint: If a mortise is worn, glue in a fitted hardwood patch, re-cut the mortise once cured, and fit a fresh tenon for a tight joint.

With careful layout, sharp tools, and patience, mortises deliver clean hardware installs and long-lasting joinery that stands up to daily use.