Picture frame mats and mat sizes

A mat is the cheapest thing in framing that makes the largest visible difference. It is also the part people get wrong most often, usually by buying a mat described by the wrong one of its two dimensions.

The two numbers on every mat

Every mat has an outside size and an opening size, and they do different jobs.

The outside size must equal the frame size. A mat for a 16x20 frame is cut 16x20 on the outside, so it sits in the rabbet exactly as the glazing does.

The opening size describes the artwork, not the hole. A mat listed with an 11x14 opening is designed to display an 11x14 print. The hole physically cut in it is smaller — about 10.5 x 13.5 inches — so that a quarter inch of mat overlaps the print on each side and holds it.

Get these the wrong way round and you buy a mat that is either too small for the frame or too large for the picture. It is by far the most common ordering error.

Standard ready-made combinations

These are the pairings you can buy off a shelf. Anything else is a custom cut, which at a framer costs perhaps fifteen to forty dollars depending on size and board, and takes a few days.

Choosing a border width

The border is the design decision. Some working rules that hold up:

Two to three inches for most things. Below about 1.5 inches the mat reads as a manufacturing tolerance rather than a choice. Above about four inches it starts to dominate, which is occasionally what you want and usually not.

Wider than the moulding. If the frame's face is 1.5 inches and the mat border is 1 inch, the two compete and the frame wins. The mat should be the wider of the two, generally by a factor of at least 1.5.

Scale with the artwork, not linearly. A 5x7 wants roughly 1.5 inches; an 11x14 wants 2.5 to 3; a 24x36 wants 3 to 4. Border width grows more slowly than artwork size.

Bigger borders for smaller, denser images. A small etching with fine detail benefits from a lot of empty space around it. A bold graphic poster does not.

Weighted mats

Cut a mat with a perfectly even border and hang it, and the image will appear to sit slightly low. This is a genuine optical effect, not a mounting error — the eye reads the visual centre of a rectangle as being above its geometric centre.

The fix is a weighted or bottom-weighted mat: the bottom border is cut a quarter to half an inch wider than the top and sides. On a 16x20 mat with an 11x14 opening, that means 2.5 inches at the top and sides and 2.875 at the bottom.

Almost all professionally cut mats are weighted. Almost no ready-made mats are, which is one of the quiet reasons a custom mat looks better even when nobody can say why.

Board types and why they matter

Standard paper mat board is wood pulp with a decorative surface paper. It is cheap and it is acidic. Over ten to twenty years the acid migrates into whatever it touches, leaving a distinct brown line — "mat burn" — along the edge of the artwork. For a poster you will replace, that is fine.

Acid-free / conservation board is buffered to neutral pH and will not burn artwork. It costs a little more and should be the default for anything you would be sorry to damage.

Cotton rag / museum board is made from cotton fibre rather than wood pulp, is inherently acid-free, and holds its colour indefinitely. This is what galleries use. It costs perhaps two to three times standard board and is worth it for original work.

Thickness is given in ply. 4-ply is the standard 1.4 mm. 8-ply is 2.8 mm and shows a substantially deeper bevel where the window is cut, which is the single most obvious visual signal of an expensive frame. It also gives more clearance between artwork and glass, which is genuinely useful for anything with texture.

Double and triple mats

A double mat is two boards stacked, the lower cut slightly larger so a narrow strip shows inside the opening. That strip — the reveal — is normally an eighth to a quarter of an inch.

Used well, the inner mat picks up a colour that already exists in the artwork and quietly reinforces it. Used badly, it introduces a colour the artwork does not contain and fights it. The rule that keeps you out of trouble: the outer mat is neutral, the inner mat is drawn from the image.

Triple mats exist. They are almost always too much.

Colour

Off-white and warm white do the overwhelming majority of framing work, for a reason that is not fashion: a near-white mat separates the artwork from the wall without adding a colour of its own, and it will still look right in fifteen years.

Bright white is harsher than people expect and can make a photographic print with warm tones look muddy by comparison. Black mats work for high-contrast monochrome work and almost nothing else. Coloured mats are best deployed as the inner mat of a double.

If you cannot decide, buy off-white. Nobody has ever regretted an off-white mat.

Cutting your own

A mat cutter is a straight guide with an angled blade holder that cuts a clean 45-degree bevel. A basic hand cutter costs about the same as four custom mats and pays for itself quickly if you frame regularly.

The technique in short:

1. Cut the outside of the board to the frame size, squarely.
2. Mark the opening on the back of the board, with the bottom border wider if you are weighting it.
3. Set the blade depth to just barely cut through — too deep and the bevel tears at the corners.
4. Cut each side with the guide, starting and stopping a fraction beyond each corner mark so the corners release cleanly.
5. Do not force a partly cut corner. Finish it with a fresh blade from the back.

The commonest beginner faults are a blunt blade, which crushes rather than cuts, and overshooting the corners, which leaves visible nicks on the face. Both are practice problems rather than equipment problems.

Mounting the artwork behind the mat

The mat holds the artwork visually; it should not be the only thing holding it physically. Attach the artwork to the backing board — or hinge it to the back of the mat — using archival hinging tape or photo corners along the top edge only, so the paper is free to move with humidity. Never tape all four sides, and never use ordinary household tape, which yellows, hardens and stains through.