Dating a la carte
From August 1864 to August 1866 photographs were taxed, which required that stamps be affixed to the photo.This has become a great way to date Civil War era photos.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.
Later photographs employed the same process, but have different names to describe their format.
Therefore, overlap in dating pictures due to image size can occur.
But combined with other features, this can be very helpful.
In 1864 two lines, a thin inner one and a thicker outer line, became popular.
In the early 1870s, much thicker lines came into vogue, and then disappeared again later in the decade.