Editing Hyperbola (section)

===Pin and string construction===
[[File:Hyperbola-pin-string.svg|300px|thumb|Hyperbola: Pin and string construction]]
The definition of a hyperbola by its foci and its circular directrices (see above) can be used for drawing an arc of it with help of pins, a string and a ruler:<ref> [[Frans van Schooten]]: ''Mathematische Oeffeningen'', Leyden, 1659, p. 327</ref>

#<li value="0"> Choose the ''foci'' <math>F_1,F_2</math> and one of the ''circular directrices'', for example <math>c_2</math> (circle with radius <math>2a</math>)</li>
# A ''ruler'' is fixed at point <math>F_2</math> free to rotate around <math>F_2</math>. Point <math>B</math> is marked at distance <math>2a</math>.
# A ''string'' gets its one end pinned at point <math>A</math> on the ruler and its length is made <math>|AB|</math>.
# The free end of the string is pinned to point <math>F_1</math>.
# Take a ''pen'' and hold the string tight to the edge of the ruler.
# ''Rotating'' the ruler around <math>F_2</math> prompts the pen to draw an arc of the right branch of the hyperbola, because of <math>|PF_1| = |PB|</math> (see the definition of a hyperbola by ''circular directrices'').