The Arabic Alphabet: A Guided Tour 

by Michael Beard, illustrated by Houman Mortazav

Ra is for Rigel (first section)

http://alifbatourguide.com/the-arabic-alphabet/ra/

Ra is proverbial for smallness (as iota is proverbial for the slight and diminutive in Greek): a simple swoop from the baseline downward. No connection on the left. It just descends, comes to a point and stops. Drawn with a reed pen it thickens half-way down and narrows again as the angle of the reed shifts. Add a dot overhead and you have the letter pronounced Z. (Add three dots and you have the sound added by Persian speakers in the 15th century, Zha.)

In their early forms the Ra and Dâl shapes look, to my eye at least, identical. Evidently early users felt the same way. Writers of Syriac distinguished them by a dot: Ra had a dot added above, Dâl had one added below. In later forms Ra (now dotless) came to descend below the base line and distinguish itself from Dâl by position. (Dâl was higher.) Dâl is often thicker at the top. Most Ra s seem aerodynamic, immersed in a right-left current which blows the thinner edges leftward. The lower point can curl up. In thulûth script Ra can extend until it almost connects on the left, because the lower point may curl up and seem to connect, as if to make the point tickle the underside of the letter which follows. In handwriting today Dal and Ra can still look very much the same.

The sound is a trilled R; when it is doubled (unlike doubled R in English, as in “current” or “barracks”), you hear the prolongation. This is the case of the two (or four) R sounds which occur in the basmala (Bism Allâh al-raḥmân al-raḥîm), where the Ra is doubled because “L-R” is pronounced as RR. (Long story. It requires knowing more about Lam.) Ra is one of those letters whose sound takes on a lot of disguises. In Sorani Kurdish it takes two identities: the familiar trilled R is written with a little V shape close underneath. The naked, stand-alone R shape is pronounced something like a glottal stop.