One of the most basic changes from primitive to Classical Adûnaic was that primitive aspirates all became spirants (SD/419). This must have been after spirant [x] became breath [h] except before [s] and for double [xx], indicating this was a relatively late change. Since primitive stops became aspirates after aspirates, these primitive stops later developed into spirants in this position as well.

In Primitive Adûnaic, stops remained where they preceded aspirates, but when these aspirates later became spirantal, in some cases they influenced preceding voiceless stops into become spirants as well, as discussed under the related rules: voiceless stops became spirants before homorganic spirants and voiceless stops became spirants before [θ].