Speffz is a standardized lettering scheme proposed by Ville Seppänen and Rob Holt in 2010. The purpose of the standardized scheme is to simplify communication between blindfolded solvers when referring to piece locations and cycles. The method is intended as an optional naming that can be used to replace the canonical notation for piece locations.
Image of a 7x7x7 cube lettered in Speffz
Specific description of the lettering
1) Pieces on a face are lettered by starting as close to the upper left corner as possible, as seen from looking at the face directly. For piece types that have two pieces close to the corner by symmetry (wing edges, t-centers), the first piece labelled is the one clockwise to the corner. Starting corners are U face - UBL; L face - LBU; F face - FLU; R face - RFU; B face - BRU; D face - DLF
2) Pieces letter around the face clockwise, when looking at the face directly, starting at the piece as stated in #1 above. The letter A starts on the U face with UBL, and the letters continue around the face in order.
3) The face order to move around the cube is ULFRBD. This proceeds in a spiral around the cube in a clockwise fashion as seen from below the cube. The first letter on the L face is E, the first letter on the F face is I, etc.
4) When lettering the D face, first perform an x2 rotation from the initial cube state, then start at what is now the UBL corner (the DLF corner as stated in #1)
5) Oblique centers on a 6x6x6 or 7x7x7 will be lettered as follows: Starting at the upper left corner of the face, the piece counter-clockwise to the corner is lettered with the appropriate uppercase letter. The piece clockwise to the corner is lettered with the appropriate lowercase letter. For the x-center orbits, and t-center orbits on a 7x7x7, you must specify whether you are talking about the inner or outer orbits.
Easy Rules to remember the lettering
1) Move around the cube in the face order: ULFRBD
2) Letter pieces always clockwise around the face
3) When lettering the obliques on a 6x6x6 or 7x7x7 letter them such as to visually appear like the following: Aa, Bb, Cc, Dd. The uppercase letters are counter-clockwise to the corner, and the lowercase letters are clockwise to the corner.
4) When labeling the D face, first perform an x2 rotation to establish the orientation of the face.