Wyckoff letter

A Wyckoff letter is the conventional alphabetic label identifying a Wyckoff position within the tables for a space group. It is normally written beside the crystallographic site multiplicity, as in $1a$. Letters begin with $a$ for a position with maximal site-symmetry group order and proceed toward the general position. The letter is a label, not a coordinate or a chemical-species symbol.^[ref]

For example, the space group $P1$ has only translations. Every point has a trivial site-symmetry group, and the general position is labeled $1a$: one equivalent position per primitive unit cell, with free coordinates $(x,y,z)$.^[ref]

An original model makes the distinction from atom counting clear. Place one species at $(0.1,0.2,0.3)$ and another at $(0.4,0.1,0.7)$ in a generic $P1$ crystal structure. Both sites are described by $1a$, but they define different crystallographic orbits. The notation does not say that the entire structure contains only one atom per cell.

The space group, its setting and coordinate convention must accompany the letter. The same letter in another group need not denote the same symmetry or coordinates. Nor does matching multiplicity by itself identify the Wyckoff position, since distinct positions can have equal multiplicity.