A page of printed Church Slavonic carries marks over almost every word: an acute, a grave, a curved mark that looks like a small arch, and, over an initial vowel, a hook. A reader coming from the manuscripts is entitled to be surprised, because the earliest Slavonic books have almost none of this.
The system is a later importation and a later systematisation. Greek manuscripts and Greek printing carried breathings and accents, and the South Slavic orthographic reforms of the fourteenth and fifteenth centuries brought a comparable apparatus into Slavonic, which the printers then regularised because type demands a rule where a pen can hesitate. The result is a scheme in which the acute is used in most positions, the grave at the end of a word, and the arched mark on certain forms where two words would otherwise look identical.
The marks are therefore not a record of ninth century Slavonic prosody. They are a reading aid, made for a language that was by then nobody's mother tongue, in books meant to be sung and chanted aloud by clergy who needed to know where the stress fell without having to know the grammar.
That is worth saying plainly, because the marks are sometimes read as evidence for the accentuation of Old Church Slavonic. For the reconstruction of early Slavic accent, the comparative evidence of the living languages is the primary witness, and the printed accents are a late and secondary one.