A glissette is the geometrical locus of a point fixed to a curve sliding inside another curve. This Demonstration shows three different glissettes that are formed by points of an ellipse sliding inside a pair of orthogonal lines (the and axes).
The blue glissette curve is traced by the center of the ellipse.
The green and red glissettes are traced by the extreme points on the semimajor and semiminor axes.
The glissette formed by the center of the ellipse is a sector of a circle with radius because the orthoptic curve of an ellipse is a circle. The orthoptic curve of an ellipse is the locus of the points from which the ellipse can be viewed under a right angle, in this case the two orthogonal axes.