The spelling of the word "THEI" is based on the phonetic transcription in IPA (International Phonetic Alphabet). In IPA, "TH" is represented by the θ symbol, while "EI" is represented by the eɪ symbol. Combining these symbols gives us "θeɪ", which is pronounced as "th-ey". Therefore, the correct spelling of the word is "THEI". The phonetic transcription helps to ensure the correct pronunciation of words, especially for those with non-standard English accents or for those learning English as a second language.