SV: Disappearance of 'a' in compounds

As a rule vowels are short before consonant clusters. The example mentioned
by Jim is canonical and occurs invariably in the same context. It is easy to
understand why the early compilers of the canon preferred saattha to the
expected form sattha because the latter would be indistinguishable from
sattha < Sanskrit s.astra "knife, sword." Another interesting example is
sadattha < Sanskrit sva + artha. Pali disallows the cluster sv and elides
/v/ because /s/ stands higher in the hierarchy of sonorities than /v/. The
expected result would therefore be sa + attha > sattha. For the sake of
semantic distinctiveness speakers preferred to maintain sa + attha and
inserted an onglide /d/ before attha. The onglide /d/ is frequent in the
canon, cf. e.g. the noun phrase ya.m ya-d-(f.acc. sic!) eva.

Ole Pind