S 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

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

X alternations

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').

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/.

Table with examples forthcoming Faytak 23:14, 14 December 2012 (UTC)

Historical conjecture

It is possible that the present-day Sereer phoneme s represents a convergence of two historical fricatives, <nonwiki>*</nonwiki>s and <nonwiki>*</nonwiki>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.