Word final L tones become H before a next-word-initial H except at the boundary of the noun phrase subject and the verb phrase, where there is sometimes a special L tone arising from the phonological clause tone pattern.

The newly-raised H can still raise the previous H in the word to S due to rule-ordering. First, the L causes the previous H to become S, then the L becomes H due to the next-word-initial H.

There is an automatic upstep rule whereby an H tone will be raised prior to an L tone within the phonological clause.
(HL --> SL)

An H tone will be raised before an L tone which has been elided. When two vowels come together at a word boundary, the first vowel together with its tone is elided.

H followed by the sequence:

1. a vowel marked with an L tone,
2. a word boundary,
3. and another vowel marked with an L tone

becomes S_ L, with the vowel and tone before the word boundary having been elided (represented as _).

The genitival relationship between two nouns is marked by an initial raised H tone on the second noun.
(Two monosyllabic genitival nouns placed next to each other with tone sequences L and H would become L and S)
(Two disyllabic genitival nouns placed next to each other with tone sequences LH and HH would become LH and SH)

In a verbal clause, the final syllable of the noun phrase object is raised before the next verb if the verb starts with an H tone.
(A noun with a tone sequence MM prior to a verb with sequence HHL, for instance, would raise its final H to become HS)

Categories of polarity, tense and mood require an upstep of an H tone.

The positive subjunctive has a raised H tone (raised, as always to S) on the final syllable of the noun phrase subject.

The negative indicative has a raised M on the final syllable of the noun phrase subject.

The future and negative non-indicative have a raised M tone on the initial syllable of the verb phrase.

In a phonological clause, an H may not be raised if there has already been another H raised, until an actual L (not an elided L, like in the general upstep case) has interposed.

(HL --> SL even if the the L was an elided L, but
HL HL --> SL HL if first L was elided
or HL HL --> SL SL if the first L was not elided)

Entry is incomplete.

Ten Nigerian Tone Systems by Elaine Thomas - 1974
document type: book
journal: Studies in Nigerian Languages
number: 4