A triangle is any set of three points on a plane and the lines connecting those points to each other, as long as the three points aren’t all on the same line (that would just be a line). Or, you could think of a triangle as the part of the plane that lies inside those line segments. A triangle is flat – it has no thickness. But every triangle has a perimeter and an area, and three angles.The three angles of a triangle will always add up to 180 degrees, no matter how big or how small the triangle is. Think of it this way: a rectangle has four 90 degree angles, or right angles. Adding those four 90 degree angles together shows us that a rectangle has 360 degrees. But a triangle is half of a rectangle, split from corner to corner. So it has to have half the degrees of a rectangle, or 180 degrees. Because of this, if you know the measurements of two angles of a triangle, you can always figure out how big the third angle is by adding the two known angles together and subtracting that from 180 degrees.