Illustration showing a circle with an inscribed triangle and radii.

Illustration showing a line with a midpoint drawn from a given exterior point to a given circumference.

Illustration showing a line that remains parallel to a given line, and touches at one end a given circumference.

Illustration showing a line that moves so that its ends constantly touch two fixed lines which are perpendicular…

The straight line is the simplest type of locus and the simplest first degree equation.