The external measurement of a frame is irrelevant. It is important to know if you have the measurement of the sight size or the rebate size (see glossary).
Between the 1620s and the 1690s, the basic range of British standard sizes was established in London: the half-length, the three-quarters, the kit-cat and the whole length. The standard size for many English portrait pictures is still the kit-cat size of 91 x 71cm (3ft x 2ft 4in).
The kit-cat takes its name from the portraits of the members of the Kit-cat Club painted by Sir Godfrey Kneller in the early 18th century.
For further reading try The National Portrait Gallery’s The Art of the Picture Frame database by Jacob Simon
A selection of antique frame firms
Isaac & Ede
Julia Korner Fine Art
■ Auction houses:
Parker Fine Art Auctions
Plowden & Smith
Everett Fine Art