Fokker block
A Fokker block (or periodicity block) is a periodic constant-structure scale that can be thought of as a region on a lattice of pitch classes (of a JI subgroup or a regular temperament) shaped as a parellelogram, parellelepiped, or higher-dimensional analog whose vertices fall upon the lattice with one vertex at the origin. A Fokker block comprises those intervals in the lattice which fall inside the parellelepiped or on the faces of the parellelepiped which intersect the origin and no others (or equivalently, those intervals which fall inside the parellelepiped after it is moved a very small amount while keeping the origin inside it). The scale repeats at the interval of equivalence, which lies on the unison in the lattice of pitch classes. If the edges of the parellelepiped correspond to intervals which are too large, the Fokker block will not be constant structure and hence a weak Fokker block.
The concept of the Fokker block was developed by the physicist and music theorist Adriaan Fokker.
Theory
Fokker blocks have a shape which tiles the lattice; an interval between pitches which lie across an edge of two Fokker blocks within the tiling will be altered from its normal value by an interval corresponding to an edge of the parellelepiped. This edge turns out to be the difference between intervals that span the same number of steps in the scale, and so it is called a chroma.
The rank of a Fokker block is the rank of the underlying lattice of pitches including the interval of equivalence. A rank-n Fokker block has n-1 chromas: a consequence of this is that a Fokker block of rank n has maximum variety at most 2(n − 1) (since that's the number of combinations of chromas a note can be altered by). For example, a rank-2 Fokker block has max variety at most 2 (hence is a mos), and a rank-3 Fokker block has max variety at most 4. In this way, Fokker blocks generalize mos scales.
Fokker blocks may be used to describe scales within JI subgroups or regular temperaments, or to describe rank-1 regular temperaments - that is, equal temperaments - themselves (by taking the chromas as commas to be tempered out).