Mats function in two ways; they provide a decorative border around the work, and they lift the glazing above the work, creating air space. This is important, as changes in heat and humidity could damage the work if the glazing is in direct contact with it.
Photographs in particular should never be in contact with the glazing, because the emulsion can adhere to it, and is not easily removable without damage.
In picture framing, matting is common on 'paper art' such as prints and watercolors. It used to be that matting material was made of wood pulp,
which is extremely acidic. These mats caused "acid burn" on artwork where the acid mixed with air and light. A brown line, sharp on the mat side and fading out toward the art's edge characterizes the burn. Old framed artwork can also be burned by lignin-bearing wood pulp backings, such as corrugated cardboard.
The standard in matboards for museums remains white or off-white 100% cotton rag matboard. There are also "alpha cellulose" matboards which are acid-free, and some of which feature a "pollution-absorbing" technology.
Fine art is secured to the backing board with small tabs or hinges of Asian paper using wheat or rice starch. There are many different types of Asian papers that can be used to secure a work in a frame depending on the weight and size of a particular piece.
Mylar photo corners are another way of securing art in a frame without attaching anything to the art. These are little paper pockets made of folded paper and secured to the substrate (mat) to hold the art in place. This method is good or art that is being shipped because it will not allow the art to swing or move out of place.