The set square (actually geometry triangle ) is a combination of ruler and protractor in the form of a right-angled, isosceles triangle . Today it is primarily an aid for drawing and math lessons , which is used there especially in the sub-area of geometry for measuring and drawing angles and making it easier to draw parallel straight lines . For technical applications, particularly high-quality set-up triangles are manufactured, which are sold under the name TZ-Dreieck ( TZ for technical drawing ).
Geo triangles are available with and without a handle on the top, with and without raised points on the bottom (so-called ink nubs ) and in different sizes, which are differentiated according to the length of the hypotenuse . This longest side of the set square is also called the ruler edge ; it has a centimeter graduation with the zero point in the middle. This is where the center line , drawn perpendicular to the edge of the ruler, begins , which the drawing device divides into two halves and shows the height with which right angles can be drawn precisely. Lines parallel to the edge of the ruler are worked into the triangle. Along the legs of the triangle there are markings at a distance of one degree from one another , the graduation runs from 0 ° to 180 ° or 180 ° to 0 °. The angle scale allows the construction of an angle with an accuracy of about half a degree.
Draw along the edge of the ruler. In particular, the right angle at the apex of the cathetus of the set square is unsuitable for drawing; at best, the squareness can be checked quickly. To draw orthogonals and perpendiculars, on the other hand, you place the center line on the given straight line and draw along the edge of the ruler. When drawing other angles, proceed accordingly: The zero point and the angle mark are on the given leg, the free leg is drawn on the edge of the ruler.
In addition to the right and half right angles, the angles 7 ° and 42 ° (or 138 ° and 173 °) are specially marked on some set square. This facilitates axonometric representation according to ISO 5456-3 ( dimetry ).
An instrument similar to the set square was used by the Egyptian geometers at the beginning of the third millennium BC. Used as a so-called construction remnant for the field measurements required after the annual Nile floods.
- Historical protractors on the website of the mathematics didactics of the University of Würzburg
- ARISTO - company / history . Retrieved February 7, 2013.
- Thomas Bergmayr: The Geodreieck comes from Tirol in: Der Standard, Journal No. 9291, September 11, 2019, p. 15.