A study in linguistic variation, this work deals with those much discussed but little researched adverbs which in present-day English sometimes occur with the suffix –ly and sometimes not, e.g. direct/directly and wrong/wrongly. The forms studied are 20 de-adjectival adverbial pairs which occur fairly regularly as verb modifiers without displaying well-established and clear semantic differences between the two forms. The present work (in two volumes) first investigates which factors have been mentioned previously as explanatory for the choice of form and then tests to what extent these factors seem to apply. Both British and American material is analysed, collected from machine-readable corpora and through elicitation. The results show, contrary to some claims, that no one parameter seems to influence the choice of form and that the adverbial lexemes vary considerably in their profiles in this respect. The lexemes can be grouped in various ways, which may be useful e. g. in the teaching of English. Some findings indicate instability, suggesting potential language change.