Transitional Horizons -dominated by properties of one master but some properties of other -AB (mostly A but some properties of a B), EB, BC.
Combination - distinct properties of two master horizons (separated by a /) - A/E, B/E, B/C.
Many combinations, sequences allowed:
Entic Haplorthod: Oi-Oa-E-Bs1-Bs2-BC-C
Typic Haplorthod: Ap-E-Bhs-Bs-BC-C1-C2
Refer to chapter 3 of the soil survey manual for complete rules of naming and numbering horizons, it can get quite complex.