A not so brief explanation of my use of accent marks
Since the use of accent marks in some of my conlangs may be a bit original some explanation of it may be in order. Basically it employs the three most common accent marks, the acute ( ˊ ), the grave ( ˋ ) and the circumflex ( ˆ ) according to the following pattern (exemplified on the letter a):
The impetus for the system comes from the fact that the circumflex graphically looks like a combination of the acute and the grave:
Use of accents in natlangs
There are several conflicting uses of the acute accent mark in Western orthographies: in some languages (e.g. Spanish, Portuguese, Catalan, Galician, Welsh and (optionally) Russian) it is used to mark the stressed vowel of a word. In other languages it is used to mark vowel length (orthographically in Irish, Hungarian, Czech, and Slovak; in scholarly use also in Old Norse and Old English).
I used to think that by contrast the grave, which is used to mark stress in Italian and Greek, was never used to mark vowel length. Scottish Gaelic proves me wrong, but on the other hand Welsh actually uses the grave to explicitly mark a stressed vowel as short!
The weakness of the system clearly is that AFAIK no language except Ancient Greek actually uses the circumflex to indicate the combination of stress and length, although it is used to mark vowel length in the orthography or transliteration of several languages. I think, however, that the graphical similarity of the circumflex to a combination of acute and grave outweighs this consideration.
Unicode and the macron
- There are still many people using email clients that don't support Unicode, while most email clients nowadays support "Latin-1" which contains the five vowel letters a, e, i, o, u with acute, grave and circumflex.
- The vast majority of Unicode-supporting fonts and applications don't handle the placement of accent marks above a letter which already bears a macron with any elegance; the likelihood is that you don't see the added accent mark at all because it gets entagled in the macron (if this ā́ actually comes out right in your browser, things may be getting better! ☺), and āˊ with the spacing acute is a kludgy substitute.
- Also — and that's actually a big part of it for me — macron and acute are notoriously difficult to keep apart in handwriting, not to speak of the combination of them, which either looks like a double acute accent (which means something entirely different in its only natlang use), or it looks monstruously clumsy.
Finally I have to admit that in the old spelling of one of my conlangs, Knòškè I exploit yet another use of the grave, namely to indicate vowel openness (as in French, Italian and Catalan): è is [ɛ] and ò is [ɔ] — against e [e] and o [o] —, but the acute still indicates length — which makes ê [ɛː] and ô [ɔː]; I guess nobody is perfect! ☺
^  This system of using accent marks was suggested to me by Raymond Brown. I readily and enthusiastically took to using it, and still am enthusiastic about it. To my knowledge Ray has never used the system himself (indeed nobody but myself seems to have used it), but I wish to acknowledge him as the inventor; I don't think it would have occurred to me on my own, since I was quite set in my earlier less systematic use of accent marks.
^  In modern Icelandic the acute is used in the same positions in the same words as scholars put it in Old Norse, but they no longer indicate vowel length, since the old distinction between long and short vowels has in the course of the history of the language developed into a distinction of quality, with most of the old long vowels having become diphthongs.
^  The Anglo-Saxons themselves apparently used the acute as length mark only occasionally if at all. The use by Old English scholars dates only from the 19th century, and was then probably inspired by the use in Irish and the somewhat less occasional use in Old Norse. Nowadays Old English scholars seem to prefer the macron, but I first met Old English in Tolkien when I was ten years old, and old habits die hard! ☺
^  Unfortunately Latin-1 doesn't contain y with grave or circumflex, nor æ or ø with any accent mark, and not even most Unicode fonts contain the precomposed forms of these letters with accent marks (other than ý obviously) but you can't always get everything!
^  Incidentally U+0341 : COMBINING ACUTE TONE MARK usually makes a better job of it: ā́, but with Ā́ and Ā́ (not to speak of Ṓ and Ṓ) you're usually in equally bad luck☹, and it is a hack in any case!