Illustration from 1908 Chambers's Twentieth Century. <b>R</b>, the eighteenth letter in our alphabet, belonging to the class of liquids--the 'dog's letter' (<i>littera canina</i>), from the trilling or vibration of the tip of the tongue.
Rev. Thomas Davidson 1856-1923 (<i>ed.</i>)
Chambers's Twentieth Century Dictionary of the English Language