When you are going to cut a picture mat board one of the things most people overlook is the squareness of the matting. Before you start to cut the mat board opening you need to ensure that your mat board is square and sized correctly. Squaring up a mat board is the first step in picture framing when you are framing a photo or framing a picture under glass.

If you can imagine when we’re framing a picture to go into a window mat board, the hole in the mat board needs to be smaller than the size of the picture itself because if you cut the mat board exactly the same size as the picture, the picture is going to fall through the mat or fall through the window in the mat. What we usually do is overlap the picture by 5mm or a quarter of an inch on each side.

If you’re going to use a tape measure to measure your picture, it is unreliable to use the end of the tape measure. I would generally come into the 100mm mark and then take 100mm off my measurement. Now on some of the rulers there is actually accurate measurements where there is a zero. If you’ve got a ruler, like a Logan mat cutter ruler, it has a clear zero mark and so therefore you can use the zero to start measuring your picture.

Squaring the mat board is simple if you follow these steps.

You will need a good aluminium or steel ruler, a good quality utility knife with snap-off blades, some scrap matting for cutting on and a sharp pencil or use a 0.5mm mechanical pencil.

Draw two diagonal lines on the back surface of your mat board. It doesn’t matter whether or not they intersect the corners so long as there are two straight lines that cross somewhere in the middle of the board.

Measure out along one of the diagonals from the centre of the board, where the lines cross, to a point near the corner of the board. The measurement is not important but you should try to measure as far as possible and pick a whole number that is easy to remember. Do this for each side from the centre along each diagonal. Mark the same distance from the centre each time.

Join up these measurements with a straight line. Do this for the perimeter of the board and you should have a squared up mat ready to measure out to the correct size.

Measure out the external size of the mat that you require using the squared up lines as your starting point.

Trim the board to size along the lines drawn using a utility knife. Make sure that you put your straight edge on the side of the board you wish to keep. This helps to protect the board because if you slip it will be into the waste and not into the board you want to keep.

Remember that the utility knife cuts along its flat surface not the tip so you should have a shallow angle between the knife and the board. That way you are presenting the maximum cutting surface of the knife to the mat board.

Another tip is to make sure you are cutting along the back edge of the ruler and not the measuring side. When you do this you will keep your ruler straight over the years rather than becoming worn by repeated trimming with the utility knife.

It is easiest to cut the board in several passes.

Score the board first and then gradually cut deeper with each successive cut. It is not necessary to cut through the board in one go unlike when you cut the bevel mat window.

When you have trimmed the squared up board you can proceed to draw up your window size on the back of the mat by measuring in from the edge of the board.