This article is about the Greek letter. For other uses, see Rho. Rho (uppercase Ρ, lowercase ρ or ϱ) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Semitic Rêš "head" (see Resh). Its uppercase form is not to be confused with the Roman letter P.
Rho is generally classed as a liquid (together with lambda and sometimes the nasals mu and nu), which has important implications for morphology. It is pronounced similarly to the letter r in languages with a Latin-derived alphabet. In polytonic orthography a rho at the beginning of a word is conventionally written with a rough breathing (equivalent to h) — ῥ — and a double rho within a word is written with a smooth breathing over the first rho and a rough breathing over the second — ῤῥ — apparently reflecting an aspirated or voiceless pronunciation in Ancient Greek, hence the various Greek-derived English words which start with rh or contain rrh. The name of the letter is written in Greek as ῥῶ (polytonic) or ρω/ρο (monotonic).
Letters that arose from rho include Roman R and Cyrillic Er (Р).
Common notational uses
The characters Ρ, ρ and ϱ are also used outside its Greek alphabetical context in Science, and Mathematics.
- In Economics to represent the discount rate of future pence cash flows
- In Molecular Biology to represent the Rho protein responsible for termination of RNA synthesis. In such occasions, it is often represented as ϱ ("rho symbol" U+03F1), to avoid confusion with the Latin letter p
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