Measuring art and frames: sight size, overlap and the parts that stack up

A frame is a stack of parts, and each part has a size derived from the one before it. This chapter measures the art, sets the overlap, names every layer and works out how deep the frame has to be.

1. Five sizes, not one Understand

People ask what size a picture is and expect one number. A framed picture has five, and each one is derived from the one before it. Getting them in the right order is most of what this chapter is.

The chain only runs one way. If you start from the frame, which is what the ready-made route does, you run it backwards and let the borders absorb the difference. What you do not do at either end is change the sheet. Trimming art to fit a frame is the one move this manual never offers, and it is irreversible in a way nothing else in the package is.

Two more words you will meet on an order form. Sight size means what you can see through the window, so it is the window opening. Rabbet is the step cut into the back of the moulding that holds the package, and a framing tool maker defines it exactly that way: the lip and inner sides of the frame that accommodate the framing materials, with the inside frame measurement called the rabbet size. Reading a framer's order form shows both words in place on a work order.

2. The overlap, and why the window is smaller

The window is cut smaller than the sheet on purpose, so the mat lies over the edge of the paper and holds it down. That overlap is the number people get wrong, because it is small and nobody tells them it exists.

How much? Nobody publishes one figure, and the ones that exist were measured for different reasons. Convention A conservation institute says the window must overlap the edges of the artifact by at least 2 to 4 mm in order to hold the work down. One US frame maker publishes its own house rule: it cuts the exact mat opening a quarter inch smaller for artwork from 4 by 6 up to 18 by 24 inches, which is an eighth of an inch of overlap on each of the four sides, and half an inch smaller above that size, which is a quarter inch per side. The UK trade specification gives 5 to 8 mm, but that figure is about something else: it is the overlap of the hinge onto the back of the sheet, not of the mat onto the front.

This manual's default is an eighth of an inch, or 3 mm, per side. That is our convention, chosen because it matches the one maker's published practice at household sizes and sits above the conservation institute's minimum. It is not a rule, and three cases change it:

• A signature or an edition number below the image. You want those visible, so the window is cut to the sheet rather than to the image, or the bottom of the window drops to include them.
• A plate mark on an intaglio print, or a deckle edge. Both are part of the object, and both are usually shown, which means the window is larger than the image and the piece is floated rather than overmatted. When a mat is wrong covers float mounting.
• A large sheet. The same maker's published practice goes to a quarter inch per side above 18 by 24 inches, because a bigger sheet moves more and a bigger window is harder to keep square.

3. Nominal against actual

A frame sold as 16 by 20 does not have a 16 by 20 opening. Its rabbet measures about 16 1/8 by 20 1/8 inches, and a framing tool maker states exactly that figure. Product-specific The extra eighth is clearance, and it is there for a reason you can measure at home: board swells with humidity. One practitioner's supplement, hosted by a national library, reports board expanding an eighth of an inch over a 40 inch length with a ten percent rise in relative humidity. A package cut to fill its rabbet exactly in January will press on the moulding in July.

So the instruction is: cut the glazing, the mats and the backing to the frame's stated size, not to its rabbet. A preservation center puts the tolerance the other way round and says the window mat and the primary backing board should be the same size and should fit the frame with a sixteenth to an eighth of an inch of total play, so the boards can move with humidity without pressing on the rabbet.

This is also why art cut exactly to a nominal size does not fit a nominal frame. An 18 by 24 inch sheet in an 18 by 24 inch frame has nowhere for the mat to overlap it; the sheet is the full size of the opening. You need a larger frame with a mat, or a float mount, or a different frame. The nearest ready-made size does that arithmetic.

4. Depth: the stack from front to back

Depth is the dimension that rules frames out, and it is the one people check last. From the front, in order: glazing, window mat (one, two or none), spacer where there is no mat, the art, the back mat or mount board the art is attached to, the backing board, and the dust cover across the back of the frame.

The only published figure in this research for the whole package on paper comes from a conservation institute: the frame's rabbet must be deep enough for the mat, the backing board and the glazing together, about 2 cm in total. The same note adds the warning that explains half the failed home framing jobs in existence: metal clip-together frames are often not deep enough to accommodate all three.

Two published board specifications sit under that. A national library and a preservation center both treat four-ply as the floor for a window mat and a backing board. The UK trade specification states its minimums in microns instead: at least 1100 microns for the mount and at least 1500 microns for the backboard. Those are not the same measurement, and the manual does not convert one into the other and pretend they agree.

The depth box adds up whatever you tell it is in the package, reports the total against the 2 cm figure, and flags two things: whether the package is deeper than the rabbet you typed in, and how much space is left in front of the art. That second number is the one to watch. Physical principle Every publisher in this research says nothing inside the frame may touch the glazing, and that the gap is made either by the thickness of a window mat or by a spacer.

5. Measuring well enough

Three habits are the whole method.

Measure twice, in the same units, and write both. If the two readings differ, you learn something: either you mis-set the tape or the sheet is not the shape you thought. Work to the sixteenth of an inch or to the millimeter. Framing is an arithmetic job and small errors compound through five derived sizes.

Measure both diagonals. Corner to opposite corner, twice. If the two diagonals differ by more than about a sixteenth of an inch, the sheet is not square, and that changes the job. An out-of-square sheet in a square window shows a wedge of white on one side, which no amount of care in the cutting will hide. The usual answers are a float mount, which makes the sheet's own edge the line you see, or a window cut to the sheet's actual shape by a framer who can see it.

Measure the frame, not the picture of it. If you are buying ready-made, put a finger in the rabbet in the shop and measure its depth with the end of a tape. A frame whose rabbet is a quarter inch deep is a frame for a photograph in a thin mat, not for a package.

7. See it See it

• Frame types, a framing tool maker: the rabbet defined, the nominal-against-actual figure, and the trade's standard size list.
• About our standard mat openings, a US frame maker: one shop's published overlap practice, in numbers, with worked examples.
• Crop ratios, a photographic lab: which print sizes share an aspect ratio, which is why a camera file does not fit an 8 by 10 frame.
• Preservation matting for works of art on paper, a practitioner supplement hosted by a national library: the clearest account anywhere of what the board trade names mean, plus the board movement figure behind the eighth of an inch of play.

8. Practice Practice

Minimum: one piece measured, entered in the calculator, and the four sizes written down. Time: about 30 minutes for three pieces. Materials: a steel tape or a long ruler marked in sixteenths or millimeters, a pencil, a clean flat surface, and the piece.

1. Measure the sheet. Width at the top and at the bottom; height at the left and at the right. Four numbers, not two. If any pair differs, the sheet is not rectangular and you have just learned the most important thing about this job.
2. Measure the image. Note how far it sits from each edge of the sheet. This is what tells you whether the piece wants equal borders or a window dropped to show a signature.
3. Measure both diagonals. Write both. More than about a sixteenth of an inch apart means out of square.
4. Put the numbers into the size calculator. Add your overlap and three border widths, and write the four sizes it returns into Your pictures.
5. Add up the stack (Go deeper; optional). Use the depth box with the package you expect, and measure the rabbet depth of any frame you are considering.

9. Your pictures Your pictures

10. Check your work Check your work

Criteria

• Four width and height readings per sheet, not two, and both diagonals.
• Sheet size and image size both written down, in the same units throughout.
• An overlap figure chosen on purpose, with a reason if it is not the default.
• Window, mat outside and frame sizes written in the picture list, not held in your head.
• The rabbet depth of any candidate frame measured and compared with the stack.

Worked example

Yusuf Ferrante (fictional) measures the 22 by 28 inch embroidered cloth from his grandmother. The textile itself is 22 by 28 at its outer edge, but the weave is not square: his two diagonals differ by nearly half an inch. That single reading changes the job. A window cut square over a cloth that is not square will show the discrepancy along one edge, and in any case the cloth has to be stitched to a covered support before it is framed at all. His measurement step ends with a note in the picture list, not with four sizes: not square, diagonals differ by 7/16 inch, support and stitching settle the size, ask a textile specialist. That is a complete result.

Common mistakes

• Measuring the image and calling it the size. The sheet is the thing that has to be held. The image is where you decide what to show.
• Forgetting the overlap. A window cut to the sheet size has nothing to hold the sheet with, and the piece drops behind the mat.
• Cutting glazing to the rabbet rather than to the frame's stated size. It will not drop in, and a package with no play presses on the moulding when the board swells.
• Buying the frame before checking its depth. Depth is the dimension that rules frames out, and it can only be fixed later by adding wood to the back.
• Trimming the sheet. There is usually another answer: a different frame, different borders, or a float mount.

What is still unresolved

• How wide the borders should be and which board to cut them from: Mats.
• Whether the stack needs a spacer instead of a mat: Holding the art.
• Whether a ready-made frame is the right route at all: A framer or yourself.
• What the whole thing will weigh once it is glazed: Weight and hardware.

11. Knowledge check and scenario

Checklist

• Sheet and image measured four ways each, with both diagonals, and written down.
• An overlap chosen on purpose, and the window size derived from it.
• Window, mat outside and frame sizes in the picture list.
• The rabbet depth of any frame I am considering measured against the stack.
• The nearest ready-made size looked up and the borders it would give written down. (Go deeper; optional)