Short answer: framing and light guidance uses words that look alike and mean different things, such as acid-free and lignin-free. This page defines 16 of them as the live guides report them, from the Library of Congress, NEDCC, the Canadian Conservation Institute and others, so a brief or a board specification can say exactly what it means. The size terms, such as rabbet and sight size, are in the frame size and profile terms.
| Term | Meaning | Where it applies |
|---|---|---|
| Lux | Lux measures how intense the light is on a surface. | Light |
| Lux-hour (lx h) | One lux of light falling on a surface for one hour. Total light dose is lux multiplied by hours. | Light |
| Overmat (window mat) | A board with an opening cut in it that sits over the print, its edges overlapping the sheet. The Canadian Conservation Institute says the overlap should be at least 2 mm to 4 mm. | Mounting |
| Float mount | The whole sheet sits inside the opening, with its edges visible. NEDCC recommends folded hinges (v-hinges) for float mounts, and the Library of Congress says overmatting protects more than floating. | Mounting |
| V-hinge (folded hinge) | A folded hinge recommended by NEDCC for float mounts. | Mounting |
| T-hinge | A hinge the Canadian Conservation Institute calls the strongest and most versatile for standard matting; it says tabs should not be narrower than 1.5 cm. | Mounting |
| Wheat starch paste | The paste the Library of Congress and NEDCC name for Japanese tissue hinges; the Canadian Conservation Institute recommends it for hinging. | Mounting |
| Mounting corners and strips | Non-adhesive supports that hold a print without hinges; the Library of Congress prefers non-adhesive hinging whenever possible. NEDCC calls self-adhesive photo corners problematic. | Mounting |
| Lignin | A component of plant cell walls. A conservation glossary cited on our guide describes it as unstable, light sensitive and breaking down into acid compounds as it ages. | Board |
| Lignin-free matboard | Matboard made from 100% cotton rag, or from chemically purified wood pulp from which the lignin has been removed, as the Library of Congress lists it. | Board |
| Acid-free | Usually taken to mean a pH of 7 or above, which may only have been true at manufacture. Acid-free is not the same as lignin-free. | Board |
| Cotton rag board | Mat board made from cotton rag fibre. NEDCC recommends 100% cotton rag mat board for every window mat and for every layer of a layered mat. | Board |
| Buffered and unbuffered matboard | Most mat boards are buffered with an alkaline material such as calcium carbonate, according to NEDCC; unbuffered boards are available for sensitive photographic materials such as cyanotypes. | Board |
| Ply | The unit in which the guidance states mat board thickness. The Library of Congress names 4-ply as a minimum and the Canadian Conservation Institute recommends 4-ply; NEDCC says thicker mats are required for large sheets. | Board |
| Photographic Activity Test (PAT) | A test the Library of Congress asks matting board for photographs to pass. | Board |
| Alpha-cellulose | A purified polymer derived from wood pulp, according to one supplier glossary, which adds that products using it are not automatically acid-free. | Board |
Back to the main private label glossary.
Sources: lux-hours and how light dose adds up on a wall [S36]; conservation matboard and mounting for framed prints [S38]; overmat vs floating mount [S41]; hinges, corners and adhesives for mounting a print [S42]; lignin-free and alpha-cellulose in matboard [S43]. As of 2026-11-12. Terms are typical and are confirmed in your written quote.