While studying at the Clayworks at Binghamton, N.Y. I would flippantly say why make one when you can make two with twice the effort. This evolved into my series called families and often use the Fibonacci series of numbers to determine family size. 0,1,1,2,3,5,8,13,21 … [Count the various sections in the large white Family of Five at the top.]
It seems natural for parents to associate members of the set with members of their own families attributing a personality to each. Within each family, the individual pieces are similar yet distinctive.