One of the problems with the grouping of music into genres is that it is
a subjective process that has a lot to do with the individual's personal understanding
and way of listening to music. This is especially true in sub-genres.
One example is Led Zeppelin, which could be called heavy metal, hard rock,
classic rock, folk, or blues, depending on one's interpretation
(and not helped by the fact that they made excursions into other genres
such as electric folk). Another difficulty with grouping artists into genres is that,
for many, their style of music changes over time. Some genre labels are quite vague.
Many were originally contrived by marketing executives or music critics;
post-rock, for example, is a term devised and defined by Simon Reynolds.
Another example of this is video game music, which while defined by its media,
can also represent its own style, as well as that of any other musical genre.