While I don't specialize in period correct historic costumes, I take them very seriously and do my best to provide a pattern that will be useful to the people involved in reenactments.
So, when asked to make a new 1860s dress pattern, I wanted to make a day dress, because I know that is what is worn most.
I searched on line in various library archives and costumer's blogs for fashion plates and photographs from the Civil War until I found a pair of dresses that were different from anything else Simplicity had published AND could be made with mostly the same pattern pieces. There is only so much room in a pattern envelope and this is an issue I constantly have to deal with.
Here is the Godey's Lady's Book image I used from 1863.
Wouldn't it be fun to make every dress in this illustration!