Six years ago, Tamura Jones at Modern Software Experience discussed Adoption and Genealogy in the posts Adapted Ahnenlist and Adoption in Genealogy. At that time, I had no idea adoption was in my future.
If you're already familiar with the traditional Ahnenlist/Ahnentafel format, his idea for adapting it to include both (A)dopted and (B)irth parents is so natural, you probably don't need me to explain it here, or follow the links in the paragraph above. An instance where the letters in a numbering system are actually meaningful in themselves is rare, it seems like kismet when it happens.
Of course, adoption isn't the only non-traditional relationship that the genealogist has to think about today. For example, it's 'conceivable' someone might want to indicate surrogacy in a chart. Polyfamilies may also wish to chart multiple lineages. The individual genealogist might need to come up with their own lettering system, but no one should feel compelled to ignore a lineage in their reports important to them in their research.
While it is possible to separate the lineages into separate reports, and this may seem easier, sometimes the whole can be greater than the parts. An interwoven ahnentafel might reveal commonalities and differences previously unconsidered. There are 26 letters of the Latin alphabet, and their meanings can be indicated in a key at the bottom of the list. If somehow we run out of room for relationships, there's always the Greek alphabet waiting in the sidelines.