>>281556895
you have four plumb lines all at right angles, if you want to make a straight line you align two of them and sight down it, then get the stake holding guy to move left and right until he's exactly lined up with the two strings, repeat until you have a few different posts
Then you can make a square corner by doing the same thing in the other direction with the other two strings if you want
it's apparently called a groma.
