Every spoken language sometimes creates new words by taking two words as free morphemes. Like in Polish, in Polish Sign Language (PSL), a compound is a rule-governed process. In author's view the PSL can create new signs - compound formation - by putting together signs that already exist in the language. It is noticed that when two signs come together to form a compound, predictable changes take place as a result of morphological and phonological rules, just as they do in Polish compound formation. Historical changes in compounds also show that PSL has a tendency for signs to become single units.