If you see violets growing in the wild, on a woodland walk in Wales, say, the species in question is most likely to be the common dog-violet (Viola riviniana). It’s odd that the called it both common and dog, given that dog attached to a name, particularly a plant, viz. dog rose, usually implies the former. V. riviniana is widespread and grows in woodland, on grassland, heaths, in hedgerows and in old pastures, flowering from April to June. This specimen I photographed in Powys, Wales in late April 2017.
One thing V. riviniana lacks is a scent, unlike its obviously named cousin, V. odorata, the sweet violet, which was used by the ancient Greeks as a perfume and even in mediaeval Britain as a deodorant.