Halliday & Matthiessen (1999: 569):
It is only in more recent times that the more covert areas of grammar have been systematically studied — those that, following Whorf, we have referred to as "cryptogrammar". Whorf (1956) distinguished between overt and covert categories and pointed out that covert categories were often also "cryptotypes" — categories whose meanings were complex and difficult to access. Many aspects of clause grammar, and of the grammar of clause complexes, are essentially cryptotypic. It is the analysis of some of these more covert features embodied in the everyday grammar, in particular the theory of mental processes, that throws light on the domain of cognitive science.