This topic is divided into lateral bend, side lean, fore and aft bend and rake.

Masthead Rig - Lateral Bend

Since sails are designed to work with a mast that doesn't bend sideways it is obvious that any sideways bend needs to be eliminated. Sideways bend on a masthead rig is often caused by too loose or stretchy upper shrouds. This is not always easy to spot.  Check the  angle of the upper windward shroud against the upper 6th of the mast and see if it has altered to a narrower gap as in the diagram below.

As the bend increases the angle between the upper shroud and the mast narrows, increasing its leverage over the shroud. once the angle decreases to less than 11 degrees, the danger of ripping the shroud out of the mast or crumpling the spreader is acute. As well as this, side bend allows the head stay to sag excessively in a breeze, when it is most undesirable.

Masthead Rig - Side Lean

Masthead Rig - Fore and Aft Bend and Rake.