1. VOWEL LENGTH MARKING IN LATIN

2. HOC·EST·FACTVM·MONVMENTVM  MAARCO·CAICILIO  HOSPES·GRATVM·EST·QVOM·APVD  MEAS·RESTITISTEI·SEEDES  BENE·REM·GERAS·ET·VALEAS  DORMIAS·SINE·QVRA

3. Inscription of the 1st or 2nd centrury A. D
Inscription of the 1st or 2nd centrury A.D.  from Nîmes (ancient Nemausus) with length shown by elongation of `i’ and apex on other vowels. For example, ÓRNÁMENTIS

4. THE ANCIENT EVIDENCE
The epitaph of Marcus Caecilius, which uses AA and EE for the first vowels of Marcus (Mārcus) and sedes (sēdēs), probably dates from the late 2nd century B.C, though some experts think it was later, with other archaic spellings (e.g. quom for cum ) deliberately adopted.
(See the discussion at )
Quintilian (1st century A.D.) tells us that old writers indicated long vowels by doubling the letter or by placing an apex over it.
The use of the apex (similar to an acute accent) continued throughout the classical period but a long `i’ was usually indicated by elongating the letter instead or by the use of `ei’ for long `i’ as in the final syllable of resistei (resistī) in the Caecilius inscription (See )
Some manuscripts, including a papyrus written around 500 A.D. and bearing lines from Juvenal, make spasmodic use of lines similar to macrons (see It is possible that these were in origin apicēs (acutes) which came to be written flat rather than rising

5. QUINTILIAN’S ADVICE
` adponere apicem ... interim necessarium, cum eadem littera alium atque alium intellectum, prout correpta vel producta est, facit: ut 'malus' arborem significet an hominem non bonum apice distinguitur, 'palus' aliud priore syllaba longa, aliud sequenti significat, et cum eadem littera nominativo casu brevis, ablativo longa est, utrum sequamur plerumque hac nota monendi sumus.’ (Institutio Oratoria, 1,7,2)
`..it is sometimes necessary to add an apex when the same letter produces a different meaning according to whether it is short or long: for example, whether `malus’ signifies a tree [málus, i.e.mālus, -ī m, apple-tree] or a man who is not good [malus] is distinguished by the apex, `palus’ has different meanings when the first syllable is long [ie. pālus, -ī m, stake] and when the second is [i.e. palūs, palūdis f, marsh], and when the same letter is short in the nominative case and long in the ablative, we should generally be advised by this diacritic which one to assume.’
Both before and after Quintilian’s time, however, writers’ actual practice was inconsistent.

6. MODERN PRACTICE
Until the late 19th century, vowels were normally left unmarked and learners were expected to learn the right pronunciation by listening to their teacher. Now, however, most books for beginners (though not printed editions of classical authors) normally mark long vowels with a macron (¯ ) or, less frequently, a circumflex (ˆ). 
Although ancient writers like Quintilian and Aulus Gellius sometimes discuss specific cases, our main source of information on vowel length is Latin poetry, which depends on a fixed arrangement of long and short syllables. However, when a vowel is followed by two consonants the syllable is normally counted as long whether the vowel itself is long or short and so we may be uncertain of the classical pronunciation. For this reason, many books omit the macron in such syllables, even in those cases where experts are fairly sure what the length should be.
Some final vowels could be pronounced either long or short (e.g. mihi or mihī) and in such cases authors may opt for one or the other or mark the vowel as anceps (i.e with both a macron and a breve (˘) over it.)
Whatever method an author or editor adopts, mistakes are very easily made and should be watched out for but viewed charitably. N.R.T. Oulton’s words, though written in jest, convey a serious point:
It is a brave man who inserts a macron over a vowel in a book which is to be read by people outside his immediate family. Or chooses not to do so, for that matter. Just think of the hoots of derision that will be heard, ringing through the countryside, as the errors are spotted and chortled over. ( So You Really Want to Learn Latin, Bk 1, p.101).