"Twins are regular aggregates consisting of crystals of the same species joined together in some definite mutual orientation."

from: "Fundamentals of Crystallography", edited by C. Giacovazzo, Union of Crystallography, Oxford University Press 1992

Simple example for a two-dimensional twin:

 
 
 
 
 

Twin Law:

fractional contribution k₁ for twin domain 1:  5/9

fractional contribution k₂ for twin domain 2: 4/9