What makes B grade different from other architectural grades?
  B Grade is intended for use where only one face and one edge are exposed. Unless otherwise specified, characteristics which do not interfere with the intended use are permitted on the reverse side and other edge. This grade also permits a cutout of not more than one lineal foot in pieces ten feet and longer for removal of defects not otherwise permitted in the grade.

for more information on redwood grades view the Adobe Acrobat version of Redwood Grades & Uses