Tone-based classification rules for Igbo nouns need modification because: (1) class 1 nouns (monosyllables with high tones) do not, as claimed, operate differently from other terminal high-tone nouns; and (2) class 6 nouns (di-syllabic with downstep tones) can be accounted for within class 2 and class 3 nouns known as HH and LH nouns). The proper nouns usually assigned to class 6 are best understood as vestigial sentences functioning in "subject position nouns." In double-object Igbo constructions, downstep occurs only in nouns occupying object-position 1 of a single object noun phrase (NP), or occupying object-position 2 of the double-object NP, and as N2 in the associative construction. If in the associative construction second-syllable high tone nouns in N2 become downstep and second-syllable low tone nouns in N2 retain their inherent low tones, the two case-tone classes serve as case-shift markers for the Igbo noun. Case-tone A occurs as subject of verb or verb-phrase, provided the noun in not part of a NP, and after an infinitive construction, even if the noun is part of a NP. Case-tone B occurs as the immediate object in an imperative verb/verb-phrase, and after a head-noun in an associated construction. (Author/MSE)