BSD, or beat seat diameter, is the ISO standard way of stating wheel sizes.
When buying replacement tyres, they must be exactly the right size for the wheel. The BSD measurement is a way to be sure they are made to the correct size.
Tyres have a pair of numbers printed on the sidewall, e.g. 28 – 622. The second of these numbers is the diameter in millimetres of the thin steel hoop embedded in the tyre edge. This steel hoop forms a ‘bead’.
Most wheels will have the BSD printed on the rim label (pictured) or occasionally embossed on the rim.
When a tyre is fitted to a wheel rim, the bead sits in a small groove in the rim known as the ‘bead seat’.
559mm is the BSD for 26″ mountain bike wheels.