Dating a la carte
In 1864 two lines, a thin inner one and a thicker outer line, became popular.
This has become a great way to date Civil War era photos.
The carte de visite changed consumer photography as much as the introduction of any other type of photograph.
First, the image had a more natural appearance than the black-base of the tintype.
For example, early cabinet cards used the exact same process as the CDV, but were just larger. Note the size may vary up to 1/4", especially with early samples as the photographer may have cut his own card stock.
But generally they were close in size, partly due to the standard size of album slots.
Also, some cards were hand trimmed by the owner, usually at an angle for easy placement in albums.