A chalk line, held taut and snapped onto a surface, will mark a perfectly straight line — up to a point. But beyond 16 feet or so, it's difficult to get the string tight enough to snap a crisp, accurate line.

The trick to marking long layout lines is to double-snap the string. Here's how: Stretch the string tight and tie it off. Then move to the midpoint of the string and hold it down with your thumb. Now snap one side to mark half of the line. Repeat on the other side to complete the line. Be careful the string doesn't move as you reach over to snap the second half.

If you've got someone to help, you don't need to tie off the string. Just pull it taut and have the other person double-snap it from the middle.
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