In Vietnamese one can decrease the effect of an adjective by doubling it. For example, to say something is “vui” means it’s fun but if you say “vui vui” it’s only a little fun. Vietnamese adjectives can be doubled in this way without any changes it the tone is ngang (no tone, as in “vui”) or the falling tone, thanh huyền.

For words with other tones (sắc, hỏi, ngã, nặng) the tone is lost in the first word. For example “đỏ” (red) becomes “đo đỏ”.

For words that end in consonant that are stops (-p, -t, -ch, -c) the tone will always be sắc or nặng and the first word will actually change. First, the tone changes. If the tone is sắc then the first word will have a tone of ngang. Otherwise, if the tone is nặng then the first word will have a tone of huyền.

The second change is to the consonant. T changes to n, p changes to m, ch changes to nh, and c changes to ng.

So for example “mát” (cool) doubles up to become “man mát” or “khác” (different) becomes “khang khác”.

This lesson in the Vietnamese language seems to suggest that in that language is a phonological relationship between the stops (plosive consonants) above and the nasal consonants they change into.