Different Systems of measurement of angle

There are three standard systems for assigning a numerical measure to an angle.

1. Sexagesimal system. The most familiar system: a right angle is divided into $90$ equal parts called degrees; each degree into $60$ equal parts called minutes; each minute into $60$ equal parts called seconds. A degree, minute and second are written $1^\circ$, $1'$ and $1''$. ("Sexagesimal" refers to this base-$60$ subdivision — the same base-$60$ idea used for measuring time.)
2. Centesimal system. Here a right angle is divided into $100$ equal parts called grades, each grade subdivided into $100$ minutes, each of those into $100$ seconds. A grade is written $1^g$. This system fits the decimal number system neatly but is rarely used in practice. …