The pronoun idem/eadem/idem appears to be the combination of is/ea/id and -dem. I can understand why isdem > īdem in the masculine, but why do we have iddem > ĭdem? Shouldn't the vowel be longer to compensate for the lost consonant? The first syllable in ĭdem is short (or light). Is the shortness there just to disambiguate it from the masculine? Such ambiguity would be easy to live with, so I expect the reason to be different. Or is idem perhaps pronounced as if it was iddem but still spelled with a single D? Otherwise the pronoun behaves as expected, but the neuter singular nominative and accusative strikes me as odd. I don't know much about length compensation, so I am certainly missing something here.