I think there's some confusion here in terms of "8 in 24". It depends on what type of carrier (i.e. flag, domestic, or supplemental) you happen to work for. (Or if you're flying a Part 91 leg for a fractional in another scenario.)
However, sticking with Part 121, in a traditional domestic scenario, you must be able to "look back" over the last 24 hours and not exceed 8 hours of flight time. (It is possible to exceed "8 in 24" due to weather or other issues once blocked out, but you can not accept a trip which, as SCHEDULED will cause you to exceed 8 in 24.) Clear as mud, right?
The irony here is that as a supplemental carrier, FedEx for example, you actually can: "Pilot may be scheduled to exceed 8 hours during 24 consecutive hours...IF he has received an INTERVENING legal rest period or twice the block hours flown since his last legal rest period.." (i.e. fly for five hours, rest for ten hours, and fly four more)
The differences hold true for Flag carriers also. I'm not going to talk anymore about this subject because it makes my head hurt, but it all depends on WHO (or is it whom?) you're flying for. (A perfect example is JB looking for an "8 in 24" exemption to fly trancon [domestic carrier]).