Families of Four-Point Touching Equilateral Triangles
This Demonstration shows four-point touching families of equilateral triangles, where , .[more]
The parameter , where is the smaller diagonal of the rhombus.[less]
A family is said to be -touching provided each element of has a nonempty intersection with precisely other elements of . A -touching family is said to be point- touching if any two touching sets have only a single common point .
 B. Grünbaum, "Families of Point-Touching Squares," Geombinatorics, 12(4), 2003 pp. 167–174. (Mar 27, 2019) sites.math.washington.edu/~grunbaum/Familiesofpointtouchin.pdf.