1d20 is rolled, and compared to the final value. If the roll is equal to, or under, the final value, the roll is a success. The difference between the roll and the final value, on a successful roll, is referred to as the “Margin of Success.” This is used in determining the overall degree of success. A high “Margin of Success” is very good, a low “Margin of Success,” while still a success, made it by the skin of your teeth.
A roll of a 1 on the 1d20 roll is always a Critical Success. This grants benefits which vary depending on what you were rolling for. A roll of a 20 on the 1d20 roll is always a Critical Failure. This causes complications and difficulties which vary, depending on what you were rolling for.
a comment from the designer of Hot Chicks
Bounty Head Bebop uses a different variation of the Inverted Twenty, differentiated with the system name Inverted20 or I20. In this variation the object is to roll under but as close to your TN (Target Number) with rolling your TN being a critical. A Roll of 1 is only a quirky success. Bounty Head Bebop's variation of this system also what is called an X-Roll to help determine damage, with the one's digit of the roll adding to a static damage number. There are several more mechanical differences between the two systems but both use a roll under mechanic.
