I've just run across a nomenclature for coincidence lattices at grain boundaries which uses a capital greek letter sigma followed by an integer. For example A.D. Rollett's 2016 Grain Boundary Engineering & Coincident Site Lattice (CSL) Theory (also archived) includes references to $\Sigma 3$, $\Sigma 5$, $\Sigma 7$ and $\Sigma 13$.
- Is it possible to explain what this means and how to know which sigma applies a given grain boundary?
- Does this apply to 1D grain boundaries between 2D grains as well? (e.g. adjacent islands of graphene side-by-side on a supporting surface)