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EulerEnvelope

Many familes of lines form an *envelope*, that is, a curve each of whose points is a point of tangency with one of the lines. Here, you see such a family of lines, each being the Euler line of a triangle *DEF* that is similar to and inscribed in the given triangle *ABC*.
The set of all triangles *DEF* "twirls" as point *D* moves on side *BC*, and the Euler line of *DEF* twirls through the envelope of tangent lines.

For more, see **GEOMETRY IN ACTION**, Chapter 8, Project 5.