Birkin

The Birkin began with a conversation on a flight in 1984, between Jane Birkin and Jean-Louis Dumas, and a sketch of a bag that would hold a week rather than an evening. Four decades later it remains the least changed of the great handbags: a rectangular body, a flap with two straps, a turn-lock closure and a lock with its own keys in a leather clochette.

Size is the first decision and the one buyers most often revisit. The 25 is the modern favourite and reads as a handbag. The 30 is the working size that still carries a day. The 35 was the original proportion and has returned to favour with the wider silhouettes now worn. Beyond them sit the 40 and the Shoulder Birkin, longer and flatter, cut to sit under the arm rather than in the hand.

Leather changes the bag more than colour does. Togo, the grained calfskin that makes up the greater part of what we hold, is hard-wearing and keeps its shape while staying soft. Epsom is stiffer, embossed and holds a crisp corner, so it photographs sharply and resists slouching. Clemence is heavier and drapes. Swift takes colour more deeply than any of them and marks more readily. Chevre is light and lustrous; Barenia patinates, and is the leather collectors argue about most.

Hardware is gold or palladium, with rarer brushed and black finishes appearing in special orders. The blind stamp gives the year of production, and we quote it wherever the leather still carries it legibly.

Condition is where price is decided. Corners, handle bases and the area beneath the flap wear first, and the interior tells you how a bag was carried. Our guides to choosing a Birkin size and what moves a Birkin's value go further; every listing here states the condition plainly rather than grading it away.