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What is a passe-partout? Mat board, bevels and the 18% rule

A passe-partout — mat, matboard, mount — is the card border between a picture and its frame. It looks like decoration. It is actually doing three jobs, and the details that make it work are small enough that most people never consciously notice them.

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What it is for

Three things, in order of how old the reason is.

  • It keeps the artwork off the glass. Paper touching glass traps moisture and eventually sticks. The mat's thickness is an air gap.
  • It gives the image somewhere to end. A picture that runs to the frame feels cramped; the border is visual breathing room.
  • It makes a small picture look deliberate. A wide mat around a modest print reads as a decision rather than a shortage of wall.

The bevel

Mat board is typically 1.4mm thick, and the window is not cut straight down — it is cut at 45 degrees. That exposes the board's inner core, which is usually white or cream even when the surface is coloured.

That thin angled edge catches light along the top and left, and falls into shadow at the bottom and right. It is a tiny detail, perhaps a millimetre and a half wide, and it is the single strongest cue your eye uses to decide that something is matted rather than simply cropped. A straight cut looks flat. A bevel looks cut.

The 18% rule

Here is the part that surprises people: a well-cut mat is not symmetrical. The bottom border is wider than the top and sides, conventionally by around 15–20%.

The reason is that optical centre and geometric centre are not the same place. A picture centred by measurement appears to sit low in its frame, because we read the bottom edge as the base of the object. Making the bottom border wider pushes the image up slightly, and it then looks centred. Frames hung above eye level exaggerate the effect, which is why gallery framing leans on the convention harder than high-street framing does.

Colour, and why it is nearly always off-white

Conservation mat board comes in hundreds of colours and professional framers use perhaps six. Warm white, eggshell, antique cream, charcoal, a muted sage, a greyed blue. The reason is that the mat is not meant to be seen — it is meant to separate the picture from the wall. A saturated colour competes with the artwork and dates badly. Off-whites do neither.

Acid-free matters too, for anything you care about. Ordinary cardboard yellows and burns a brown line into the paper it touches within a decade or two.

Doing it on a screen

All of the above is why a television showing a photograph edge-to-edge does not look like a framed print, no matter how good the panel is. There is no border, no bevel, no depth. Art Frame TV rebuilds these in software for Google TV and Android TV — a 45° bevelled core, bottom-weighted borders, and the depth shadow the mat's thickness casts onto the print beneath it.

Common questions

What is a passe-partout in framing?
A passe-partout is the card border placed between artwork and the frame — also called a mat, matboard or mount. It holds the artwork off the glass, gives the image visual breathing room, and makes a small picture look deliberate.
Why is mat board cut at an angle?
The 45-degree bevel exposes the board's pale inner core, which catches light along the top and left and falls into shadow at the bottom and right. That thin lit edge is the main visual cue that a picture is matted rather than cropped.
Why is the bottom of a mat wider than the top?
Because optical centre sits slightly above geometric centre. A mat with equal borders makes the picture look like it is sliding downward, so framers make the bottom border roughly 15–20% wider.
What colour should a passe-partout be?
Almost always an off-white — warm white, eggshell or cream — or a deep neutral. The mat's job is to separate picture from wall, not to be noticed, and saturated colours compete with the artwork.

Art Frame TV puts your photos and gallery classics inside photorealistic frames with real passe-partout matting, on Google TV and Android TV. Free, no account, no ads. See how it works →

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