A Mat Board is a cut-out piece of coloured cardboard that is placed around an art piece, usually in conjunction with a picture frame. It is used to enhance the art piece it surrounds, compliment it, or to bring out features that would not otherwise be seen or emphasised.
The Mat Board is frequently made out of a specialised type of card - it should be inert, that it does not effect the artwork it is surrounding in any way. It should be acid-free, should not readily absorb moisture, and should not readily fade even in direct sunlight.
Mat Boards come in a variety of colours, in fact covering the entire spectrum, although the most used colours would include shades of white and light browns.
Specialised mats might have a gold or other metallic covering so that they reflect a greater amount of light. Mat boards can also be found that have a different colour inside that compliments or contrasts the main colour.
Mat Boards are traditionally cut at a 45-degree angle so that the interior white (or other colour) can be seen. Adding to the framing effect of the mat board and frame.
Mat boards should be carefully chosen. When gotten wrong they can distract the viewer of a piece of art. An understanding of art itself is often required - there are some pieces of art that you would never frame with a bright colour, others that should only be pastels and so on. Keeping in mind, of course, that the best judge of the right colour to use will lie in your own judgement of what you think looks best and what effect you are going for.
For example, if you want a picture to stand out in a room, you might use bright colours that will draw a viewer into the picture. You may want to emphasise a particular portion of the artwork, in which case a matt matching that colour will make it pop (you would be surprised how well this works), or if it is a more traditional piece, the matt would compliment the general colour theme.
A good picture framer will have what they call corner samples. These are small pieces of mat board in the various colours available. By placing these to the corner of your artwork you (and the framer) can judge the right mat board for your use. Use these corner samples in conjunction with corner samples of the frame you are planning to use so that you can see the whole effect clearly.
What pieces of art do you have around that could be enhanced by a mat board?