The /v/ sound is spelled with <v> or <ve>. The /ð/ sound is spelled <th>.
The contrast is between a labio-dental fricative and a dental and alveolar fricative, both voiced. The sounds are so similar that they cause problems even to native speakers, and a common error among children is to write or say "cloves" for "clothes". An uneducated speaker in a novel might be shown as saying "I won't breave a word about it, guv." Luckily the contrast is so rare that it causes few problems. A number of the pairs involve rare or obsolete words.
Interesting pairs include:
fervour further
never nether
sliver slither
The mean density value is 0.9%, giving some support to the O'Connor conjecture. The list makes 22 semantic distinctions, a loading of 73%.
breve breathe breves breathes clove clothe cloves clothes fervour further hive Hythe Iver either lave lathe laves lathes live lithe loaves loathes never nether oaves oaths privy prithee reeve wreathe reeves wreathes sheaves sheathes sliver slither slivered slithered slivering slithering slivers slithers V thee V's these vale they'll van than vat that vats that's vie thy vine thine vow thou
John Higgins, Shaftesbury, October 2010.