Roman emperors Constantine VIII and Basil II, reigning for 66 years in total (962–1028) and for 65 years in total (960–1025) respectively, are not included, because for part of those periods they reigned only nominally as junior co-emperors alongside senior emperors.
A distinction is not made between absolute and constitutional monarchs, hence Elizabeth II is listed second despite being a figurehead her entire reign.
Bolkiah will be eligible for inclusion if he is still reigning in 2040 and surpasses Conrad I's independent rule of 56 years, 99 days.
In a given year, there may have been a wide array of actual reign lengths based on days.
The list is limited to those that might reasonably be expected to lie within the range of those in the tables above, at minimum 56 years.