The spelling of the word "Willbur" can be explained using IPA phonetic transcription. IPA represents sounds in language, allowing for accurate spelling. In "Willbur," the first syllable is pronounced as /wɪl/, with the "i" sound being represented by the IPA symbol /ɪ/. The second syllable is pronounced as /bər/, with the "u" sound represented by /ʌ/. Therefore, the correct spelling is "Wilbur," with an "i" and "u" vowel sound in each syllable.