S and X Alternations

The consonants /s/ and /x/ behave unexpectedly with regard to the consonant mutation processes in Sereer: they are the only mutating consonants that do not consistently maintain their original place of articulation when mutated, moving (in the case of /s/) from an alveolar to a palatal place of articulation in certain cases, and (in the case of /x/) moving from a uvular to a velar place of articulation at times.

S alternations

A major exception to the regularity of the standard nominal mutation, s is sometimes invariant and sometimes not. When it alternates under the standard nominal mutation, it does so with the voiceless palatal stop c in certain lexemes and is invariant in others.

English gloss	Singular	Plural
pile of millet husks	soxon	coxon
village	saate	caate
onion	soble'	soble'
boat	suk	suk

Parallel to this, the prenasalized nominal mutation produces diminutive and augmentative noun stems in nj (e.g. suukar 'sugar'; njuukar 'sugar-DIM, a bit of sugar').

X alternations

The segment x is never invariant, but mutates to either nq or ng depending on the lexeme along essentially the same lines of variation as /s/.

English gloss	Singular	Plural
lightning	xiiñ	xa-kiiñ
year	o-xiid	xa-qiid
half-bundle	xumb	qumb
to berate	xas	nqas
to make a mistake	xas	ngas

Historical conjecture

It is possible that the present-day Sereer phoneme s represents a convergence of two historical fricatives, *s and *ʃ. The lexical items beginning with s that now undergo standard nominal mutation may have originally been a palatal segment, explaining the changing place of articulation. Comparison of related languages will no doubt indicate the veracity of this conjecture.

Similarly, it is possible that modern Sereer /x/ is a merger of two historical phonemes, one of which formerly had a regular mutation to nq and the other to ng. Obvious candidates for these merged segments are suggested by the two guttural phonemes in Sereer Siin, /x/ and /h/, which have merged to /x/ in Sereer Salum.