How do you keep string line straight?

I'm setting patio stones along a wood fence which is unfortunately
wavy and not straight.I'm trying to keep the patio stones in a straight line with a white string attached to sticks which a planted on the ground just above the patio stones to go straight.Since I cannot rely on the fence which is crooked,how do I go about on keeping the string straight?Is there a method,mathematical or not to make sure the string is
going straight.

Answers:
determine the length of your proposed patio. At one end measure out from your wavy fence the distance of your patio blocks plus one inch. At the other end do the same. Now check from the string to the fence along the string, to make sure that your blocks will fit all the way. If so, then use this line as the edge of first course of patio block. Next courses should be laid along the first using it as your straight edge. (your string should be attached to a stake at each end, and stretched tightly)

Other answers:
tie both endes with a tight knot
tie both endes with a tight knot
If your fence is crooked you can just eyeball the line and adjust as necessary...make damn sure the string is tight and off the ground.
You really need a transom, laser level, or water level. Any of the three will help you set and maintain a straight line that you can then measure off of.
be sure the sticks you have the string to deep in the ground,
use a cotton string and pull it tight then tie it to the stick
you should use that like chalk like it marks a chalk line of like blue an it stays straight so you might want to think about going and purchasing it from like lowes or home depot!
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