Tightening the headstay induces forward rake, easing headstay increases aft rake. Rake controls helm. Forward rake adds lee helm. Aft rake induces weather helm. A normal 40-footer has 12 to 18 of rake.
Rake is measured with a plumb.
Rake may vary with boat type and normal sailing conditions. It keeps the mast bending positively, preventing mast inversion and reducing pumping in choppy conditions. Mast butt shape can affect pre-bend in a negative way. Set pre-bend using one of the two methods described below. The goal here is to make the mast straight sideways and minimize sag to leeward.