I read that there is a feature called Improved Critical.
>>Benefit: When using the weapon the character selected, the character's threat range is doubled.
Special: The character can gain this feat multiple times. The effects do not stack. Each time the character takes the feat, it applies to a new weapon.
Note: "Keen" magic weapons also double their normal, nonmagical threat range. As with all doubled doublings, the result is triple.
Well I found out that it increases the dice range for criticals, so a weapon that only crits at 20 would crit at 19 and 20.
But I dont know what happens to a 19-20 or a 18-20 weapon.