Having finally been convinced that D20s are archaic and 3d6 is the way of the future, the 3d6 model has been adopted for general play. This means that 18s are considered the automatic hit and natural critical threat. Bear in mind that this happens FAR less frequently. While a 20 has a 5% chance of coming up on a d20, an 18 only has about a 0.5% chance, less than on tenth as common. However, this also means that 3, the new automatic miss, has a similarly reduced chance of coming up. Probability distribution is centered much closer around 10.5, producing average results and increasing the importance of roll modifiers. 3d6s are rolled for all attack, maneuver, skill, and ability rolls, essentially replacing the d20 in its entirety.
Due to the changes with the distribution, critical threat ranges have been modified as well. Weapons that previously threatened on a 20 (nearly all) now threaten on 18 (obviously). Weapons whose range was 19-20 are now 17-18, and weapons whose range was 18-20 are now 16-18. This does not represent a linear relationship, though. 17-18 has roughly 1.9% chance of coming up, four times as often as 18, and 16-18 has a 4.7% chance of coming up. Because of this change, Improved Critical and Keen no longer double threat ranges, but instead increase them by 1. Furthermore, they also stack. A Keen rapier whose wielder has improved critical with light blades, for example, would threaten from 14-18, about 16.2% chance of coming up. This is only slightly more than the 3.5 base model without any modifiers on 18-20 (15%).
: All weapons in this group gain -1 to their minimum threat range. Stacks with the Keen quality.
: Weapons with this quality gain -1 to their minimum threat range. Stacks with the Improved Critical feat.
*: Weapons with this quality gain -2 to their minimum threat range. Stacks with the Keen quality, but overlaps with Improved Critical.
*Augmented Critical is only found on the Tarrasque. Normal play will never encounter this (unless you encounter the Tarrasque, in which case start praying/running).