Nobles were born into a noble family, adopted by a noble family (this was abolished in 1633) or ennobled by a king or Sejm for various reasons (bravery in combat, service to the state, etc.—yet this was the rarest means of gaining noble status). Many nobles were, in actuality, really usurpers, being commoners, who moved into another part of the country and falsely pretended to noble status. Hundreds of such false nobles were denounced by Hieronim Nekanda Trepka in his Liber generationis plebeanorium (or Liber chamorum) in the first half of the 16th century. The law forbade non-nobles from owning nobility-estates and promised the estate to the denouncer. Trepka was an impoverished nobleman who lived a townsman life and collected hundreds of such stories hoping to take over any of such estates. It does not seem he ever succeeded in proving one at the court. Many sejms issued decrees over the centuries in an attempt to resolve this issue, but with little success. It is unknown what percentage of the Polish nobility came from the 'lower' orders of society, but most historians agree that nobles of such base origins formed a 'significant' element of the szlachta.
What was a rarest rare to become a noble?
service to the state