each written with a single letter, . This is the reason for the terms for vowel letters described below. In grammars, textbooks, or dictionaries, are sometimes marked with macrons () to indicate that they are long, or breves () to indicate that they are short. For the purposes of accent, vowel length is measured in morae: long vowels and most diphthongs count as two morae; short vowels, and the diphthongs in certain endings, count as one mora. A one-mora vowel could be accented with high pitch, but two-mora vowels could be accented with falling or rising pitch. Monophthongs Close and open vowels The close and open short vowels were similar in quality to the corresponding long vowels . Proto-Greek close back rounded shifted to front early in Attic and Ionic, around the 6th or 7th century BC (see below). remained only in diphthongs; it did not shift in Boeotian, so when Boeotians adopted the Attic alphabet, they wrote their unshifted using . Mid vowels The situation with the mid vowels was more complex. In the early Classical period, there were two short mid vowels , but four long mid vowels: close-mid and open-mid . Since the short mid vowels changed to long close-mid rather than long open-mid by compensatory lengthening in Attic, E.H. Sturtevant suggests that the short mid vowels were close-mid, but Allen says this is not necessarily true. By the mid-4th century BC, the close-mid back shifted to , partly because had shifted to . Similarly, the close-mid front changed to . These changes triggered a shift of the open-mid vowels to become mid or close-mid , and this is the pronunciation they had in early Koine Greek. In Latin, on the other hand, all short vowels except for were much more open than the corresponding long vowels. This made long similar in quality to short , and for this reason the letters and were frequently confused with each other in Roman inscriptions. This also explains the vocalism of New Testament Greek words such as λεγεών ('legion'; < Lat. legio) or λέντιον ('towel'; < Lat. linteum), where Latin was perceived to be similar to Greek . In Attic, the open-mid and close-mid each have three main origins. Some cases of the open-mid vowels developed from Proto-Greek . In other cases they developed from contraction. Finally, some cases of , only in Attic and Ionic, developed from earlier by