[…] the ‘r’ and ‘l’, lingual semivowels, used in many languages also as vowels; the ‘l’, even in English, in ‘able’, ‘eagle’, etc. The ‘r’ is produced between the tongue tip and the roof of the mouth, and is so generally trilled or vibrated that trilling is apt to be given as its distinctive characteristic; the ‘l’ sets the tip of the tongue against the roof of the mouth, but leaves the sides open for the free escape of the intonated breath.
- Whitney (1875), a pag.66