#Ellipse

Key E. Centre and angle: the first click sets the centre, then the direction and the semi-axes.

#When you need it

An ellipse shows up where a round section is cut at an angle: a slanted branch pipe, a hole in a slanted wall, a decorative cut-out. If what you really need is a slanted cylinder, it is safer to extrude a circle and tilt the body — an ellipse in the sketch gives the same shape but loses the link to the original diameter.

#What defines it

A centre and two semi-axes, major and minor. Rotation is a separate parameter, so a tilted ellipse stays an ellipse instead of turning into a spline.

#Where it is used

Oval windows and hatches, eccentrics, transitions between round and flat. An ellipse is an exact curve, not an approximation made of arcs: a surface extruded from it stays smooth, and a machine cuts it without steps.

#Do not confuse it with a slot

A slot is two semicircles and two straight lines; its width is constant along its whole length. An ellipse narrows towards its ends. A screw guide needs a slot; a shape needs an ellipse.