Phonology as we know it from the study of spoken languages deals with the study of elements that by themselves do not carry meaning but may convey meaning differences in a language (as evidenced by minimal pairs, such as English cat – bat – mat). The parameters of a sign are also crucial for making such meaning distinctions. So, within an individual sign language, we find minimal pairs for each of the parameters (take an example from your own sign language), and this is evidence for the claim that parameters fulfil a phonological function.