The spelling of the word "corder" can be explained using the IPA phonetic transcription. This word is pronounced as /ˈkɔːdər/ in British English, and as /ˈkɔːrdər/ in American English. The presence of the letter 'o' and the combination of the letters 'er' indicate the vowel sound 'ɔː'. This is followed by the letter 'd', pronounced as 'd', and the suffix '-er', pronounced as 'ə'. Hence, the correct spelling of the word is "corder", using the letters 'o', 'd', and 'er'.