One common historical development in languages with distinctively nasalized vowels is the excrescence of coda velar nasals in place of nasalized vowels. For example, the dialect of French spoken in the southwestern part of France (Midi French) is characterized by words ending in the velar nasal [ŋ] where Parisian French has nasalized vowels and no final nasal consonant ([savɔ̃]~[savɔŋ] ‘soap’). More generally, there is a cross-linguistic tendency for the unmarked place of articulation for coda nasals, and perhaps also for stops, to be velar. In four experiments, we explored why the cross-linguistically unmarked place for the excrescent nasal is velar. The experiments test Ohala’s (1975) acoustic explanation: that velar nasals, having no oral antiformants, are acoustically more similar to nasalized vowels than are bilabial or alveolar nasals. The experiments also tested an explanation based on the visual phonetics of nasalized vowels and velar nasals: velar nasals having no visible consonant articulation are visually more similar to nasalized vowels than are bilabial or alveolar nasals. American English listeners gave place of articulation judgments for audio-only and audio-visual tokens ending in nasal consonants or nasalized vowels. In the first and second experiments, we embedded recorded tokens of CVN (N = /m/, /n/, or /ŋ/) words in masking noise and presented them in audio-only and audio-visual trials. We also synthesized ‘placeless’ nasals by repeating pitch periods from the nasalized vowel to replace the final consonant in CVm with nasalized vowel. These stimuli provide a direct test of Ohala’s acoustic explanation of coda velarity in nasals. The third and fourth experiments extended these results with tokens in which the last portion of CVN (N = /m/, /n/, or /ŋ/) and Cx̃ syllables were obscured with masking noise. These experiments were designed to force listeners to assume the existence of a final consonant and to rely primarily on visual cues in a more direct test of the visual similarity of nasalized vowels and velar nasals. Taken together, the results of these four experiments suggest that excrescent coda nasals tend to be velar because nasalized vowels are both acoustically and visually similar to velar nasals.