Refer to chapter 3 of the soil survey manual for complete rules of naming and numbering horizons, it can get quite complex.
From Soil Taxonomy:
Transitional and Combination Horizons
Horizons dominated by properties of one master horizon but having subordinate properties of another: Two capital-letter symbols are used for such transitional horizons, e.g., AB, EB, BE, or BC. The first of these symbols indicates that the properties of the horizon so designated dominate the transitional horizon. An AB horizon, for example, has characteristics of both an overlying A horizon and an underlying B horizon, but it is more like the A than like the B.
In some cases, a horizon can be designated as transitional even if one of the master horizons to which it presumably forms a transition is not present. A BE horizon may be recognized in a truncated soil if its properties are similar to those of a BE horizon in a soil from which the overlying E horizon has not been removed by erosion. A BC horizon may be recognized even if no underlying C horizon is present; it is transitional to assumed parent materials.
Horizons with two distinct parts that have recognizable properties of the two kinds of master horizons indicated by the capital letters: The two capital letters designating such combination horizons are separated by a virgule (/), e.g., E/B, B/E, or B/C. Most of the individual parts of one horizon component are surrounded by the other.
The designation may be used even when horizons similar to one or both of the components are not present, provided that the separate components can be recognized in the combination horizon. The first symbol is that of the horizon with the greater volume.
Single sets of horizon designators do not cover all situations; therefore, some improvising has to be done. For example, Argic Udipsamments have lamellae that are separated from each other by eluvial layers. Because it is generally not practical to describe each lamella and eluvial layer as a separate horizon, the horizons can be combined but the components described separately. One horizon then contains several lamellae and eluvial layers and can be designated an "E and Bt" horizon. The complete horizon sequence for this soil could be: Ap-Bw-E and Bt1-E and Bt2-C.