It commonly represents the alveolar trill /r/, like the "rolled" sound in the Scottish pronunciation of ⟨r⟩ in "curd".

It has no connection to the Latin letter P (P p), which evolved from the Greek letter Pi (Π π), despite both having the same form.

The name of Er in the Early Cyrillic alphabet was рьци (rĭci), meaning "speak".

[1] In the Cyrillic numeral system, er had a value of 100.

The Cyrillic letter Er (Р р) looks similar to the Greek letter Rho (Ρ ρ), and the same as the Latin letter P (P p; П in Cyrillic).