The center of the circle is represented by the letter O, the line segment connecting the center of the circle with any point on the circle is called radius, the line segment passing through the center of the circle with both ends on the circle is called diameter, and it is represented by the letter D. The position, radius and diameter of the center determine the size of the circle. In the same circle or equal circle, the radii are all equal and the diameters are all equal. The diameter is twice the radius, and the radius is 1/2 of the diameter.

Expressed in letters: d=2r or r=d/2.