The date and commissioning of many paintings in the past have often been established by art historians on the basis of a legal contract.
For example, a contract was frequently drawn up by an individual or an institution wanting an altarpiece. In such documents it might be specified how much of the work should be carried out by the named artist, as distinct from associates or subordinates.
In addition, the nature and the cost of materials were often specified, as well as the payment the artist should receive, and the subject.