The word prognatus
as written on the Sarcophagus of Lucius Cornelius Scipio Barbatus
(280 BC) reveals the full development of the Latin R
by that time; the letter P
at the same time still retains its archaic shape distinguishing it from Greek or Old Italic rho
The original Semitic letter may have been inspired by an Egyptian hieroglyph for tp, "head". It was used for /r/ by Semites because in their language, the word for "head" was rêš (also the name of the letter). It developed into Greek 'Ρ' ῥῶ (rhô) and Latin R.
The descending diagonal stroke develops as a graphic variant in some Western Greek alphabets (writing rho as ), but it was not adopted in most Old Italic alphabets; most Old Italic alphabets show variants of their rho between a "P" and a "D" shape, but without the Western Greek descending stroke.
Indeed, the oldest known forms of the Latin alphabet itself of the 7th to 6th centuries BC, in the Duenos and the Forum inscription, still write r using the "P" shape of the letter.
The Lapis Satricanus inscription shows the form of the Latin alphabet around 500 BC. Here, the rounded, closing Π shape of the p and the Ρ shape of the r have become difficult to distinguish.
The descending stroke of the Latin letter R has fully developed by the 3rd century BC, as seen in the Tomb of the Scipios sarcophagus inscriptions of that era. From around 50 AD, the letter P would be written with its loop fully closed, assuming the shape formerly taken by R.
Late medieval illuminated initial
18th-century example of use of r rotunda
in English blackletter typography
The minuscule (lowercase) form (r) developed through several variations on the capital form.
Along with Latin minuscule writing in general, it developed ultimately from Roman cursive via the uncial script of Late Antiquity into the Carolingian minuscule of the 9th century.
In handwriting, it was common not to close the bottom of the loop but continue into the leg, saving an extra pen stroke. The loop-leg stroke shortened into the simple arc used in the Carolingian minuscule and until today.
A calligraphic minuscule r, known as r rotunda (ꝛ), was used in the sequence or, bending the shape of the r to accommodate the bulge of the o (as in oꝛ as opposed to or). Later, the same variant was also used where r followed other lower case letters with a rounded loop towards the right (such as b, h, p) and to write the geminate rr (as ꝛꝛ). Use of r rotunda was mostly tied to blackletter typefaces, and the glyph fell out of use along with blackletter fonts in English language contexts mostly by the 18th century.
Insular script used a minuscule which retained two downward strokes, but which did not close the loop ("Insular r", ꞃ); this variant survives in the Gaelic type popular in Ireland until the mid-20th century (but now mostly limited to decorative purposes).