Question: Implicit skill on weapon 6 link?

At which skill-level do you get a 6 link on your skill which comes from a weapon. Is it 19 or 20? I have a lvl 18 weapon with a 5 link and a lvl 20 weapon with a 6 link. Its really cheap to upgrade my weapon to lvl 19 but very expensive to lvl 20 so I was just wondering.