I mean, the system is clearly related to seeing if the sides are of equal length and if the angles equal 90 degrees. If we first exclude the hexagonal, we have three sides and three angles each of which could be either equal/90degree. That gives us 3^2 = 9 possibilities. Then looking at the hexagonal and with the same logic, the last length can either equal the two other or it can not, which gives another two possibilities. The total then being 11 possibilities.. why is there for example no structure with [tex]a = b \neq c, \;\; \alpha = \beta = 90 \neq \gamma[/tex]?