Get The Fuck OutLayout & JoinerySquare with a rope

How do you lay out a true 90° without a square?

Your speed square is 180 mm across. The panel is 2400 mm across. Guess which one wins the argument about whether that line is square.

⚡ Rule of thumb

Use a rope or thin strip as a compass to strike two equal arcs from points on your baseline; the line through where the arcs cross is a true perpendicular.

Starting point

Two equal-radius arcs from two baseline points, struck with the same rope length

Measured

Perpendicular to the baseline, through the arc intersection

Confidence

Widely accepted

▶ Source video — Easy 90° Angle with a Rope — Letterman Mebelwatch on YouTube ↗

Applies to

Process: Layout — carpentry, metal, concrete, groundwork
Materials: Lumber, panels, slab, or bare dirt
Setup: Rope, string or a thin wood strip, plus a pencil or marker
Scale: Best where the workpiece is bigger than any square you own

Why it matters

This is a geometric construction, not an approximation. The angle comes out at exactly 90° whether your baseline is 300 mm or 30 m long, because the arcs do the work.

A small square referenced across a big sheet amplifies every fraction of a degree of error along the length. Extending a short square into a long line is how a wall ends up 20 mm out at the far end.

What good looks like

Both arcs swung at the same radius, with the rope at the same tension
A clean, unambiguous crossing point above the baseline
The drawn line checks out against a square where a square will reach

How to apply it

Mark a point on the baseline. Swing equal arcs to each side of it to get two marks on the line.
From those two marks, swing two more equal arcs that cross above the line.
Draw from the original baseline point through the crossing. That line is square.
Spot-check with a level or square if one will reach.

Exceptions & tradeoffs

The whole thing rests on the radii being equal. A rope that stretches under different tension on each swing quietly gives you 89°, and 89° is worse than useless because it looks fine.

Use a non-stretch line — mason's line or a wood strip beats a bungee-ish rope every time.

Common mistakes

Pulling the rope harder on the second arc than the firstHold the same tension, or switch to a rigid strip with a pencil hole
Marking a crossing so shallow you can't tell where the arcs actually meetSwing wider arcs so they cross at a steeper angle and the point is obvious

Letterman MebelSource & visual reference

Striking equal arcs with a rope to construct a true perpendicular, verified on screen with a square — Easy 90° Angle with a Rope ↗ at 0:00–0:16.

Summarized and independently edited by Start Here. Starting values and cautions are our editorial notes, not a quotation from the creator.