Q: What is the criteria for inclusion?
A: This chart prioritizes early classics instead of the "best ever" or "most popular" from each genre, since the most essential album for each genre should be the one that "started" the genre. This won't include the very first example of every genre however, just the first "popular/influential" recording for each genre. This gives us a better perspective on the history of the genre and the history of music genres in general. If you don't like this approach you could still use this chart to guide yourelf based on the genres, which you can then look them up on sites like RYM and search for the "best" or most popular album from the genre.
The exception to this rule was given when dealing with repeated genres, in which case the second spot was given to the "most different sounding" recording found from the same genre (the closest there would be to a sub-genre from within that genre), so with the intention to maximize stylistic diversity.
Traditional music does not work this those rules, and instead what we look for is the most diverse and "pure" example of its genre (unless there is an actual "first").
Some categories would be filled with singles and therefore not have enough representation in terms of duration when compared to other genres where the album format is more dominant, therefore singles might get replaced with albums when possible. I'm still unsure on this rule however.
When there is no proper first single/album, a Various Artist compilation for the early years of the genre is preferred.
Q: How is the categorization of the chart determined?
A: The genre categorization used in this chart is based on stylistic lineage. For example, Rock is all the music that evolved out of rock & roll, jazz is all the music that evolved out of new orleans brass bands (the parent genre of dixieland), and so on. This should also explain why vocal jazz is under vaudeville instead of jazz, since vocal jazz evolved out of vaudeville blues, itself derived from vaudeville. This also explains why Finnish Tango is under Latin American and not in Western European, since the roots of Tango are of Latin American origin. Similar case goes to artists of "folk" and "protest music", since those styles are based on a style of North American contemporary folk developed by Woody Guthrie out of country. The only exception to this rule was given to genres of "post-colonial music" (American genres with the exception of Native American), mostly for simplicity's sake.
Regional categories are not purely regional, but instead refer to the cultural traditions associated with that region (it's possible for a Japanese person to play Brazilian Samba, for example). This should explain why there are no recordings of rock or jazz in the regional categories.
Q: Why is there no pop category?
A: Because it wasn't needed. Is as much of a genre as "hardcore" or "fast", no genre of music evolved out of pure "pop". It's more like an adjective, if anything. It wasn't necessary to add it when it came to making the music genre tree either (on which this chart was based on), so that's another reason to not consider it a genre.
Q: What's up with the "Experimental" and "Other" categories?
A: Experimental genres are not really genres in the same way as the others, since they don't adhere to any specific style, but rely on a "universal" musical element instead (minimalism with repetition, free improvisation with improvisation, noise with noise, etc). This also explains why experimental genres don't have a proper parent genre as well, since some experimental genres have been "invented" at different places independently at times (which is something that doesn't happen with regular genres). The Other genres are proper genres (or styles) that derived from Experimental (like Industrial under Free Improvisation), or genres that were styilistically too far appart from their parent genre to be under it (like Easy Listening under Big Band).