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The semi-major axis ( denoted by a in the figure ) and the semi-minor axis ( denoted by b in the figure ) are one half of the major and minor axes, respectively.

These are sometimes called ( especially in technical fields ) the major and minor semi-axes, the major and minor semiaxes, or major radius and minor radius.

The four points where these axes cross the ellipse are the vertices, points where its curvature is minimized or maximized.