When viewing an HR diagram, the main sequence curve is apparent, and the general shape of it is obvious. However, in this truncated version, it's unclear to me exactly which stars should be considered main sequence. I've added a shaded grey area as what I think I should count as main sequence, but there's a bit of room for interpretation. Are there hard-and-fast rules, or is it a matter of opinion? What do you think of this shaded area?