In a triangle with a square corner (a right angle), each of the other two corners has a cosine. You find it by taking the side that sits next to that corner — not the long side — and dividing it by the long side (the side across from the square corner). The number you get is the cosine of that angle. Though actually, cosine is not the size of the angle in degrees. It is a ratio: one length divided by another. People still say “the cosine of the angle,” meaning that number. It is a school and science word; nobody asks for the cosine of something in a shop. Students always meet it in a set with sine and tangent. The name is a clue: cosine was short for Latin “sine of the complement.” The complement is the other acute angle that sits with it to make 90 degrees, and the cosine of one angle is the sine of that other one. In maths, people often write cos as a short form.