A wall ends up crooked, out of level, or out of plumb. Why? Most of the time it's because the mason didn't use a line or didn't use it properly.
CORNER POLES AND LEADS
To build long, straight walls, you have two options: you can build corner leads and tie the line to the leads, or you can attach corner poles to the building structure and tie the line to the corner poles.
ATTACHING THE LINE
Once the corner poles or leads are up, the line can be pulled. Attach the line to corner poles with line blocks, making sure both ends of the line are at the same course height marked on each pole. When leads are used, you have several options for attaching the line. You can use line blocks, line pins, nails, or line stretchers.
PULLING THE LINE
Usually the mason on the right pulls the line taut. He should pull it as tightly as he safely can. The tighter the line, the straighter the wall will be. If the line isn't pulled tightly enough, it'll sag and the wall will be out of level.
TAKING THE SAG OUT OF THE LINE
On a long wall (40 feet or more), it may be impossible to remove all sag in the line just by pulling it. To eliminate sag and prevent the wind from blowing the line, set one or more masonry units in the middle of the wall to hold up the line. These masonry units are called trigs. Use a metal clip called a twig to hold the line to the trig.
LAYING MASONRY UNITS
Once the line is pulled tight and the trig accurately set, you can start laying brick or block. Make each bed joint the same even thickness, and don't "crowd the line" with the brick or block.