If they are not of your dynasty then give them a title somewhere in your kingdom - this will make them your vassal. However, most of the time you will not be so lucky. Invite them to court and press their claim. If you are lucky enough that they are of your dynasty and have a strong claim you are all set. All of those with a green thumbs up will accept an invitation to your court. You can get a list of claimants by clicking on the coat of arms of the duchy you want (or a county in that duchy and then navigating to the duchy by clicking on the CoAs at the top of that screen), then clicking on the claimants button. If the current duke is adult male, they need a strong claim, otherwise a weak claim works as well. If they are not of your dynasty or your vassal, the duchy will not join your realm after the war (although the claimant still gets the title). I find that these usually work quite well.ДЄnother, potentially a lot more complicated way is to find a claimant to one of the duchies who are either your vassals or of your dynasty and pressing their claims. Sometimes you will also get plots to fabricate a claim. This option should always work, but it takes a while (unless you are Catholic and your target is the pope). Most of the time he will get a claim on a county, but there is a small probability that he will obtain a claim for the whole duchy. The most straightforward (although not necessarily speedy) way is to send your chancellor to fabricate claims.

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