The triangle is a geometric, flat figure. This means that it is completely flat and lies on the paper, so to speak. Each triangle has three sides, 3 corners and 3 angles. The picture next to it shows how everything is written down correctly.
At school, the triangle is taught in geometry. In primary school, pupils learn how to draw a triangle accurately from different data. For example, they need the three side lengths or two side lengths and an angle size. Later, more complicated constructions and even calculations are added. These were already known in ancient times and were used for all kinds of constructions.
Triangles are very important in everyday life. A triangle is always stable, you cannot deform it without bending the sides. If you lay a triangle of three matches, you always get the same shape. If you put four matches together to form a square, there are an infinite number of different possibilities.
Technology makes use of this fact: Triangles are often found on bicycles, on bridges or in a roof truss, because they are absolutely stable. A bicycle or a bridge will not collapse if they are built on triangles. A roof truss made of triangles is very stable and does not bend.
Triangles are also symbols in religion: in Christianity they represent the Trinity. This consists of God as the Father, Jesus as the Son and the Holy Spirit. The Jewish Star of David consists of two triangles: one symbolises God, the other the people.