A straightedge or straight edge is a tool used for drawing straight lines, or checking their straightness. If it has equally spaced markings along its length, it is usually called a

ruler A ruler, sometimes called a rule, line gauge, or scale, is a device used in geometry and technical drawing, as well as the engineering and construction industries, to measure distances or draw straight lines. Variants Rulers have long ...

. Straightedges are used in the automotive service and machining industry to check the flatness of machined mating surfaces. True straightness can in some cases be checked by using a

laser line level A laser line level is a tool combining a spirit level and/or plumb bob with a laser to display an accurately horizontal or vertical illuminated line on a surface the laser line level is laid against. Laser line levels are used wherever accurate v ...

as an optical straightedge: it can illuminate an accurately straight line on a flat surface such as the edge of a plank or shelf. A pair of straightedges called

winding stick An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in de ...

s are used in woodworking to make warping easier to perceive in pieces of wood. Three straight edges can be used to test and calibrate themselves to a certain extent, however this procedure does not control twist. For accurate calibration of a straight edge, a

surface plate A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup. The surface plate is often used as the baseline for all measurements to a workpiece, therefore ...

must be used.

Compass-and-straightedge construction

An idealized straightedge is used in compass-and-straightedge constructions in

plane geometry Euclidean geometry is a mathematical system attributed to ancient Greek mathematician Euclid, which he described in his textbook on geometry: the '' Elements''. Euclid's approach consists in assuming a small set of intuitively appealing axioms ...

. It may be used: * Given two points, to draw the line connecting them * Given a point and a circle, to draw either tangent * Given two circles, to draw any of their common tangents * Or any of the other numerous geometric constructions The idealized straightedge is: * Infinitely long * Infinitesimally thin (i.e. point width) * Always assumed to be without graduations or marks, or the ability to mark * Able to be aligned to two points with infinite precision to draw a line through them It may not be marked or used together with the

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

so as to transfer the length of one segment to another. It is possible to do all compass and straightedge constructions without the straightedge. That is, it is possible, using only a compass, to find the intersection of two lines given two points on each, and to find the tangent points to circles. It is not, however, possible to do all constructions using only a straightedge. It ''is'' possible to do them with straightedge alone given a circle and its center.

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Making Accurate Straight-Edges from Scratch
{{Measuring and alignment tools Mathematical tools Metalworking measuring instruments Stonemasonry tools Technical drawing tools Woodworking measuring instruments

Compass-and-straightedge construction

See also

References

External links