The spelling of the word "rulethumb" can be explained using the International Phonetic Alphabet (IPA) transcription. This word consists of two syllables, "rule" and "thumb," with the stress on the first syllable. The "r" sound at the beginning is pronounced with a slight rolling motion of the tongue, followed by the "y" sound in "you." Then, the "oo" sound in "rule" is followed by the "th" sound in "thumb," which has a dental fricative sound. Overall, "rulethumb" is spelled as /ˈruːlθʌm/.