We have a 75 hour guarantee, meaning that all pilots are guaranteed to make at least 75 hours of pay monthly (unless they drop trips to go below that).
If you are awarded a line, you also have a line guarantee. For example, if your line is blocked at 90 hours, then that becomes your guarantee for the month.
Guarantees are computed on a daily basis. Whatever flying you have blocked for the day is your guarantee. If you are schedule to fly 4:30, then you are guaranteed 4:30 of pay.
If you underblock, fly less than 4:30 (which frequently happens), then you still get paid the full amount.
If you overblock, fly more than 4:30, then your actual flight time (from parking brake release to shutdown) is what you are paid. For example, the other day had left the gate for a 0:45 minute flight when the airport shut down due to thunderstorms. We sat on the taxiway for over 2 hours and ended up going an hour above our guarantee at the end of the day, so we made an extra hour of pay.
So if you overblock on one flight, you might underblock the next. The net overblock for the day would be what you are paid.
To further muddy the waters, some work is above guarantee. If you pick up "open time" trips on your days off, then that is paid above your line guarantee.
Additionally, we also get full pay for canceled trips.