qwyx [pronounced like "quicks"] is an unofficial, ad hoc encoding for the representation of Serbo-Croatian letters with diacritical marks in electronic form. There are five such letters in the Latin script.
qwyx is based on the following idea: to represent these letters using four letters of the English alphabet, q, w, x, and y, which are otherwise not a part of the Serbo-Croatian alphabet. In this way, the "non-English" letters too become part of the lower ASCII, which permits free spreading, reading, writing, etc., of Serbo-Croatian texts -- as far as English texts go: across all platforms, through all network services and servers, to devices other than computers (e.g., cellular phones), and throughout the world -- without having to worry whether and which kind of language support is installed on the other side.
Since the letters in question are five and the English letters are four, one of them is represented by a two-letter combination. There are also schemes for intermixing Serbo-Croatian and English letters q, w, x, y, i.e. two meanings of the same symbols, for later conversion to traditional writing.
I have chosen to write this page in Serbo-Croatian and give only a brief English summary. If you have reasons to believe that I should elaborate more of it in English, let me know and state your case.