File:8 choose 2 triangle.svg

Array of the ${\displaystyle {\tbinom {8}{2}}=T_{7}=28}$ two-element subsets of an 8-element set (Binomial coefficient, Triangular number)

Each hexagon represents a set ${\displaystyle \{a,b\}}$ for which ${\displaystyle a,b\in \{0...7\}}$ with a pair ${\displaystyle (a,b)}$ for which ${\displaystyle a<b}$.

Below the two figures the same pair is indicated by highlighting two among 8 positions.

${\displaystyle (a,b)}$

${\displaystyle a}$

${\displaystyle b}$