Museums apply registration, inventory, or accession numbers to each of their objects for the purpose of identification and security. The manner in which these numbers are applied varies according to the type of object. Many different materials have been used as supports for paintings, including wood panel, canvas on stretcher, and metal sheet. Less common supports, such as ivory or reverse paintings on glass (where the paint layer is on the back of the glass), can present some difficulties when applying registration numbers. General recommendations such as those found in this Note are useful, but common sense is always most important; certain cases may require bending the rules. Some works of art, such as contemporary paintings without stretchers or paintings on glassm may present problems not covered in this Note. In these cases, consult a conservator.