Butterfly Theorem
Given a Chord 
of a Circle, draw any other two Chords 
and 
passing through its Midpoint. Call the points where 
and 
meet 
and 
. Then 
is the Midpoint of 
.
References
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 45-46, 1967.
- Steiner's Theorem
- Steiner Surface
- Steiner System
- Steiner Triple System
- Steinhaus-Moser Notation
- Steinitz's Theorem
- Steinmetz Solid
- Stella Octangula
- Stella Octangula Number
- Stellated Polyhedron
- Stellated Tetrahedron
- Stellated Truncated Hexahedron
- Stellation
- Stem-and-Leaf Diagram
- Step
- Step Function
- Step Polynomial
- Steradian
- Stereogram
- Stereographic Projection
- Stereology
- Stern-Brocot Tree
- Stevedore's Knot
- Stewart's Theorem
- Stickelberger Relation