A numeric, hierarchical knowledge classification system, (e.g., Dewey Decimal Classification (DDC)) of concepts, is used for universal annotation. For example, “game” (DDC=799) relates to hunting and “game” (DDC=794.105), to chess. The universal annotation may be combined with human-aided machine translation, so individuals can tag their documents in the source language, simultaneously specifying meanings in the source and target languages, exercising control over what is said in the target language. Multilingual communication, by e-mails, forums, blogs, Wikipedia and the like may proceed, with each participant working in his tongue and tagging his writing with universal concepts. Moreover, a Universal System of Expression (USE) is introduced, based on the universal concepts. USE documents may be generated as byproducts of human-aided machine translations between any two natural languages that recognize USE, for fully automatic translation to other languages, on demand, and libraries of USE documents may be formed.