US enlisted men's woollen trousers from 1861-1865 now show dark brown flax sewing thread on the blue cloth. This study showed -- from documentary evidence, Army specifications, and thread purchases -- that dark blue flax thread was purchased. Documentary sources also showed that three blue dyes were in commercial use at that time: indigo, Prussian blue, and logwood. Laboratory analysis of the thread from uniforms of the Civil War period showed that the dye was not Prussian blue or indigo. Considerations of the conditions of use, cleaning, and storage showed them to be similar to those needed to change logwood blue to brown. The conclusions were that the original thread was blue, dyed with logwood, and turned dark brown probably due to light and oxygen exposure. -- AATA