Lachlan's term for a set of four points, no three of which are collinear.
Tetrastigm
See also
TetragramExplore with Wolfram|Alpha
References
Lachlan, R. "Properties of a Tetrastigm." §139-146 in An Elementary Treatise on Modern Pure Geometry. London: Macmillian, pp. 85-90, 1893.Referenced on Wolfram|Alpha
TetrastigmCite this as:
Weisstein, Eric W. "Tetrastigm." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Tetrastigm.html