From H. W. Smyth, Greek Grammar, I §§149-188
GENERAL PRINCIPLES, ANASTROPHE, CHANGE OF ACCENT IN DECLENSION, INFLECTION AND COMPOSITION, PROCLITICS, ENCLITICS
ACCENT: GENERAL PRINCIPLES
149. There are three accents in Greek. No Greek accent can stand farther back than the antepenult.
1. Acute (´): over short or long vowels and diphthongs. It may stand on ultima, penult, or antepenult: καλός, δαίμων, ἄνθρωπος.
2. Circumflex (~): over vowels long by nature and diphthongs. It may stand on ultima or penult: γῆ, θεοῦ, δῶρον, τοῦτο.
3. Grave (`): over short or long vowels and diphthongs. It stands on the ultima only: τὸν ἄνδρα, τὴν τύχην, οἱ θεοὶ τῆς Ἑλλάδος.
150. The acute marks syllables pronounced in a raised tone. The grave is a low-pitched tone as contrasted with the acute. The circumflex combines acute and grave.
151. Accented syllables in Ancient Greek had a higher pitch (τόνος) than unaccented syllables, and it was the rising and falling of the pitch that made Ancient Greek a musical language. The Greek word for accent is προσῳδίᾱ (Lat. accentus: from ad-cano), i.e. ‘song accompanying words.’ Musical accent (elevation and depression of tone) is to be distinguished from quantity (duration of tone), and from rhythmic accent (stress of voice at fixed intervals when there is a regular sequence of long and short syllables).
N. – The accent heard in Modern Greek and English is a stress-accent. Stress is produced by strong and weak expiration, and takes account of accented syllables to the neglect of the quantity of unaccented syllables. Thus, shortly after Christ, ἄνθρωπος was often pronounced like a dactyl, φίλος like a trochee; and πρόσωπον, ἐννέα, were even written πρόσοπον, ἐννήα.
Reference address : https://www.ellopos.net/elpenor/lessons/greek-accentuation.asp