In the ancient Greek alphabet, three of the seven letters representing vowels were called dichrona, "of two quantities": unlike ε, η, ο, ω, which distinguished a quantity, α, ι, and υ could indicate both a short and a long quantity. Among the surviving ancient grammatical treatises on dichrona, the one attributed to Aelius Herodianus (2nd century CE) is the most famous. This volume offers a new critical edition of the treatise on dichrona attributed to Herodianus, the first to be based on its entire manuscript tradition. In the introduction, the contents of this treatise are related to other Herodianic fragments on vowel quantities (these fragments mainly come from the ancient scholia to Homer, but also the epitomes of Ps.- Arcadius and John Philoponus, the grammatical work of Georgius Choeroboscus, Byzantine lexica and etymologica, etc.), as well as to three other anonymous treatises on vowel quantities that are also related to the theory on dichrona elaborated by Herodianus (in the appendix, new critical editions of these three texts are provided, too). By comparing the contents of all these sources, it is possible to shed light on the theory on vowel quantities elaborated by Herodianus.