spelt

From Late Middle English spelde (past tense) and spelled, ispeled, ispelled (past participles). The past tense reflects an early Middle English syncopation of spellede, while the past participle remained largely disyllabic until the general Early Modern English reduction of the -ed suffix. In both cases, once the medial vowel was lost, the final alveolar stop underwent phonetic devoicing after the sonorant /l/, resulting in the modern -t spelling. Compare dwelt, felt, and smelt. By surface analysis, spell +‎ -t.