The spelling of the surname "Neidig" can be explained using IPA phonetic transcription. The first syllable "nei" is pronounced /neɪ/, with the "ei" sound representing a long vowel sound, similar to "ay" in "play". The second syllable "dig" is pronounced /dɪɡ/, with a short "i" sound and the consonant "g". Therefore, the correct spelling of the surname is "Neidig", with a long "i" vowel sound in the first syllable and a "g" consonant sound in the second syllable.