Costner sits next to 3 phonetic neighbors — Foster, Coster, and Holster are the closest matches by sound. Most share Costner's English-origin cluster, so the rhythm and vowel shape carry across the list.
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Similar sound: These names share phonetic patterns with Costner — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old English origins, giving them the same cultural heritage as Costner.
Rhythm: Costner is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.