http://web.princeton.edu/sites/ehs/scaffold/appB.htm
Bracing
Transverse bracing forming an "X" across the width of the scaffold must be installed at the scaffold ends, and at least at every third set of posts horizontally (measured from only one end), and every fourth runner vertically.

Bracing must extend upward "diagonally" to opposite sides of the scaffold, such as from the inner posts or runners to the next outer posts or runners, or vice versa.