A correspondent from abroad writes to express his puzzlement over the difference between such linguistic terms as alphabetic and alphabetical, analytic and analytical, diacritic and diacritical. He asks if these are synonyms, and if so, is there a way of deciding which to use?
The issue goes well beyond the terminology of linguistics: examples from other domains include mystic(al), poetic(al), ironic(al), philosophic(al), rhythmic(al), astronomic(al)... Usually there's no difference in meaning, but there may be a stylistic or regional preference, and only an up-to-date dictionary or usage guide (such as Pam Peters' Cambridge Guide to English Usage: see the entry at -ic/-ical) will help. Where the two forms are synonymous, people generally opt for the shorter alternative; but the extra -al syllable can sometimes produce a more euphonious utterance, avoiding a clash of consonants or promoting a better rhythm (compare geographic contours vs geographical contours).
In a few cases, the meanings have diverged: electric and electrical now have different ranges of usage, as do comic and comical and some others. The -al ending usually suggests a broader meaning - more things are 'comical' than are 'comic'. Occasionally, the meanings are very different - notably economic and economical, historic and historical, politic and political. Usage essays in dictionaries and usage guides often focus on the differences.
In a specialist domain, the only thing one can do is identify and follow majority usage. In linguistics these days it is phonetic, phonological, grammatical, and semantic (but you will find the alternatives in older usage). It is sometimes possible to tell the difference between a specialist and a non-specialist by the ending: those who talk about syntactical structures or semantical problems or linguistical issues are hardly likely to be specialists in linguistics.