A number of Latin words show -urC- where -orC- would be etymologically expected. In this article, a collection is made of the reliable examples, and previous explanations are assessed. No regular sound change that explains all the good examples exists, and it is concluded that an origin in dialectal Latin, although superficially supported by the apparent parallel of cases of -irC- for -erC-, is difficult to substantiate. Instead, there seem to be two sources: firstly, a regular Latin sound change * ΣorC- > urC-, as in *Σor-Σo- > uruum ‘plough’; and secondly, borrowing from Umbrian, where, it is argued, -ur- is the regular reflex of *-‰- (e.g. the preverb pur- < *p‰-).