The /ð/ sound is spelled with <th> or <the>. The /r/ sound is spelled with <r> or <rr>. Notice how the homograph row makes pairs with though and thou.
This is a contrast between a dental fricative and a continuant which has several allophones. They are both voiced and fairly close together in the mouth, but the contrast does not cause problems.
The contrast only occurs initially or medially, and , since /r/ is a common sound while /ð is rare, the mean density value is low at 0.2%. The list makes 16 semantic distinctions, a loading of 94%.
tether terror tethers terrors that rat thee re their rare then Rennes then wren there rare they Ray thine Rhine though roe though row those rose thou row thy rye thy wry Worthing whirring
John Higgins, Shaftesbury, November 2010.