Quote Originally Posted by joysof
the vocative: [...] It doesn't generally exist in modern Russian except in relic form, usually connected with worship: 'боже', 'господи' etc.
That's not quite true. It is used in colloquial speech fairly often. You could say: "Маш, поцелуй меня; Сань, дай закурить" instead of "Маша, поцелуй меня; Саня, дай закурить", which is formally "more grammatical". As far as I can tell you can only use it with nouns ending with -а or -я; the other declensions are represented only by the exceptional "боже, господи, отче"; another exception is "владыко", which is apparently the vocative of "владыка" that has retained the ancient declension when it is used to address a high ranking clerical. It is an interesting question whether you could say "владык" when addressing somebody who is not superior (that means it will be jocular, though, so perhaps it will work just fine).