Fairfax Classic GPs have three billets (straps), with the third one being moveable.

Classic GP Girthing

The saddle fitter will begin with the neutral position – 2nd and 3rd billets

If the saddle moves forward, they will try the 1st (point strap) and 3rd billets

Classic GP Girthing

If the back of the saddle lifts, the saddle fitter will move the 3rd billet onto the floating balance position and use the 2nd and 4th positions

For increased stability (on a very round horse for eg) the saddle fitter may use the 1st (point strap) and 4th position

DO NOT GIRTH USING THE 1st and 2nd BILLETS TOGETHER - this will cause saddle instability