Effective Clauses: The Feature ‘Agency’

Halliday & Matthiessen (2014: 349):

Strictly speaking an effective clause has the feature ‘agency’ rather than the structural function Agent, because this may be left implicit … The presence of an ‘agency’ feature is in fact the difference between a pair of clauses such as the glass broke and the glass was (or got) broken: the latter embodies the feature of agency, so that one can ask the question ‘who by?’, while the former allows for one participant only.