Every workman has experienced more or less difficulty in nailing up corner blocks in casing doors and windows. The trouble all comes from the want of a solid background on which to nail the blocks. Very often the plastering is not finished level and true with the jambs. All trouble with corner blocks may be avoided by taking a common board of the proper thickness, 1½ inches narrower than the inside head casing, 1½ inches shorter than the width of window and side casings, and nail it tight down on the head jamb, as shown in Fig. 55. By this method the corner blocks will nail up true and solid without cracking the plastering. Care should be taken that the board is not too wide nor too long, as the blocks and head casing should completely cover it from view.