The principles of phonological analysis can be applied independently of modality because they are designed to serve as general analytical tools, not language-specific ones. The same principles have been applied to the analysis of sign languages (see Phonemes in sign languages), even though the sub-lexical units are not instantiated as speech sounds.
Instead of being language-specific what kind of tools are the principles of phonological analysis designed to be? What other types of language have the phonological analysis principles been applied to? What are the principles of phonological analysis able to be applied separately from? In sign languages what are not represented as instances of speech sounds?