Herbert Weir Smyth, A Greek Grammar for Colleges

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[*] 156. The circumflex is formed from the union of the acute and the grave (́̀ = ^), never from ̀́. Thus, παῖς ῀ πάὶς, εὖ ῀ ἔὺ. Similarly, since every long vowel may be resolved into two short units (morae), τῶν may be regarded as = τόὸν. The circumflex was thus spoken with a rising tone followed by one of lower pitch. μοῦσα, δῆμος are thus = μόὺσα, δέὲμος; μούσης, δήμου are = μὸύσης, δὲέμου. In διδοῦσα (i.e. διδόὺσα) compared with διδούς the accent has receded (159) one mora.

a. The whole vowel receives the acute when the second short unit of a vowel long by nature is accented: Δί_ ῀ Δὶί.