R vs r

R and r both are nouns.

Word	Noun	Adjective	Verb	Adverb
R	Yes	No	No	No
r	Yes	No	No	No

As nouns, R and r are synonyms defined as:

R (noun)	r (noun)
the 18th letter of the Roman alphabet	the length of a line segment between the center and circumference of a circle or sphere
a unit of radiation exposure; the dose of ionizing radiation that will produce 1 electrostatic unit of electricity in 1 cc of dry air	the 18th letter of the Roman alphabet
(physics) the universal constant in the gas equation: pressure times volume = R times temperature; equal to 8.3143 joules per kelvin per mole