The Rope square is used as a reference to make a right angle (similar to a speed or combo square) the really neat part of it is you don't require any special tools or precise equipment, but by using a trick of trigonometry you can get a accurate method of gauging a 90 degree angle.

Supplies

For this you need something to reference as a length (anything will do; a ruler, a stuck, a box, your foot/hand. Here I used a old staple tin I had laying about), and a sufficently long rope/string/twine. This should be about 14 or 15 times the length of your reference to give you plenty of room. 550 cord is pretty good for this.

Step 1: Step 1

Tie a knot in your rope at least one (1) length from the end plus a few inches or centimetres.

Step 2: Step 2

Place your rope on your reference with the lead to the remainder of the rope (the long side) across the reference length. Mark or pinch the the rope on the unknotted side of the reference.

Step 3: Step 3

Knot that point of the rope, confirm the placement with the reference that the length is right.

Step 4: Step 4

Move the new knot over so there is an unknotted part of the rope across the reference.

Step 5: Step 5

Repeat steps 3 & 4 until you have 11 knots on the rope.

Step 6: Step 6

Place the first and last knot on one side of your reference with the remaining leads across the reference length.

Step 7: Step 7

Using a square or reef knot, tie the two leads together and confirm the length. this should make a loop of rope with 12 knots in total.

Step 8: Step 8: Finishing (optional)

Cut and clean the loose leads and secure your rope so it wont fray and come undone.

Step 9: Step 9 Use Your Square(which Is a Loop...that Becomes a Triangle)

Hold and secure one knot. Count 3 knots down the rope. Hold that point and pull it so the line is taunt between the two points and secure it. Continuing down the rope count 4 knots. Holding that knot pull the knot so the lines become taunt to the first and the second knots you secured. You have now created a 3-4-5 triangle, and a solid reference to a right angle (I've bought manufactured squares that were worse)

Note: This is also easy to do with a friend, one person holding the short side and another person holding the other point of the triangle.