Each of the four business letters in this group is addressed to Thomas F. Mason. Their content pertains to the trade in cotton and other commodities conducted "between the lines" during the American Civil War. While blockading the seacoasts of the Confederacy to limit international trade, President Lincoln established a system in which government licensed traders might purchase cotton in the occupied South. This was designed to bring cotton to northern textile manufacturers and to foster "Unionism" among southerners by engaging them in trade with the North; all "loyal citizens" of the Union were eligible to take part. In order to minimize southern gains from the trade, the Lincoln administration devised a new policy under the Purchasing Act of 1864, which required licensed traders to sell all their cotton to Treasury Department officials at three-quarters of the market price.

In practice, this system was plagued by corruption and cronyism. Prominent northern capitalists often used their political connections to obtain permits, and so monopolized the trade. Military commanders in occupied areas of the South confiscated cotton in areas under their jurisdiction, declaring these goods to be their own "prizes of war." Treasury agents who purchased cotton under the system set up in 1864 often manipulated the rules of trade for their own gain and favored merchants willing to bribe them. Though between-the-lines trade was meant to foster loyalty to the Union, it had the effect of boosting the Confederate economy by introducing badly needed goods to a South stifling under the shortages caused by the international blockade.

The first three letters in this group were written to Mason by an unidentified representative of J. T. Whiting & Co., a Detroit-based merchant shipping company formed in 1860 to transport goods to and from the mining towns on Lake Superior. Whether the letters were written by owner John Tallman Whiting himself is not clear. Mason utilized Whiting's steamers to carry his copper on the lakes, and as these letters show, the two were jointly involved in the Mississippi River cotton trade as well, investing in the cargos of chartered vessels that travelled south from Memphis with goods and cash and returned north with cotton.

The letters show the uncertainties of the trade, with cotton purchased from Confederate areas sometimes seized by greedy military officers, and between-the-lines commerce disrupted by Treasury officials and senior military commanders to benefit their own interests. Ships involved in the trade were also vulnerable to Confederate fire, as indicated by the destruction of "Allen Collier," one the vessels run by Whiting. The letters also indicate the great profits from the trade, and show that Whiting and Mason had powerful allies in the northern government and military who used their influence to obtain them trading permits and overcome obstruction by other officials and officers.