This paper examines Chaucer’s use of the modals moot/moste and shal/sholde based on conditional elements effective for their semantic production. The modals are semantically sensitive to three types of conditions: 1) conditions deriving from the proposition (person of the subject, lexical features of the subject, aspect of the main verb, voice of the main verb, etc.); 2) conditions of the clause structure containing the modal (the modal in the independent clause, the modal in the main clause in conjunction with the subordinate clause, the modal in the subordinate clause, the types of adverbial harmonics co-occurring with the modal); 3) the pragmatic conditions involved in the use of the modal (mode of speech, background information of the speaker and the hearer, external causals implied in the usage of the speaker, etc.). In this paper, these three conditions are applied to Chaucer’s moot/moste and shal/sholde. I have examined the extent to which those conditions are quantified, or to which they can be tagged in an electronic text, and how the semantic data of these modals can be assessed. Numerically significant differences arise in, for instance: ‘God’ subjects—moot/moste, 0/470; shal/sholde, 21/2539. External causals, however, although semantically important, cannot be easily quantified because they are so often undistinguished as well as unmarked.