string will help you to mark straight lines, define angels and so. To tighten the string one needs a rapid knot, easy to make and to undo.

Step 1: Other Tools

a length of string, slightly elastic
something to measure with
a pencil
some nails
a hammer
and possibly some tongs

Step 2: Measure and Mark

Its up to you what you want, where you need the line for, or what angels you want to define. her is just an abstract example

Step 3: Nailing

as long as they hold a little strain, don't hammer them in too far

Step 4: Fix Knot 1

On on side just a simple knot will do. you might leave it afterwards

Step 5: The Knot , Firts Step

the idea is to twist the loop 8 to 12 times, depending much on the string resistance. I mostly twist counterclock wise, doesn't seem to matter much.

Step 6: Tightening

You might want to help the tightening, pulling the fixed part of the string towards the nail, also pulling the loose end , working with both hands.

Step 7: Closing the Knot

by moving the loose end, keeping it tight, to the opposite side, the knot moves towards the nail. then let go. if all is correct, the knot stays. If it slips, try again, twisting is a couple of times more

Step 8: Ready

you now have a tight string, a straight line, mark the piece where you need it, or transport the angle to your work piece.

Step 9: Loosening

Bring the loose end again towards te tight string, and pull. The knot is gone. Get the twisted string of the nail.

Step 10: Finished

pull the nails out, roll the string, you can use it for years. String is a basic tool,
try it, the knot is made in seconds.
good luck with your work and projects.
TeKaKa