Corners From 1859 on, CDVs (Cartes de Visite) had square cut corners. From 1872 on, corners were rounded.
Card Backs In the 1860s the backs of CDVs were very thin, often they were layers of paper/cardboard. By 1870 the backs were getting thicker. By the later 1880s and 1890s the backs were very thick.
Verso (Back) The 1860s verso had a simple logo and writing, somewhat like an ink stamp. By 1865 the logo had changed to a design in the middle. By 1868 logo designs had begun to be much more intricate and by the 1870s there was usually a design within a shaped frame.
Revenue Stamps were required on photographs in the USA from 1864 to 1866. Their presence (or their absence) gives us the approximate dates of ancestor photographs of the Civil War era.
Another method of dating old photographs is to look for the Photographer's printed address on the back of the card, then research the known dates of his studios.
For more information and help dating old photographs, plus images, see
Hints for Dating Old Photographs