This is murky territory, full of confusion and jargon, with every brand, outdoor shop and expert using different terminology.
To keep it simple, here’s a guide to One Planet references.
Back length: the length of your back needs to be matched by a pack with a suitable frame length. We refer to our frames as short, medium and long.
To measure your back length, record the vertical distance from a point level with the crest of your shoulders to your iliac crest (the top of the hip bone).
It can be difficult to measure your back length accurately. To obtain a perfect fit, it is best to have a One Planet pack fitted by a One Planet dealer.