The ninth chapter discusses various topics relevant to business and trade, including barter, bills of exchange, weights and measures and bookkeeping, in 150 pages.

[1] The book's mathematical content draws heavily on the traditions of the abacus schools of contemporary northern Italy, where the children of merchants and the middle class studied arithmetic on the model established by Fibonacci's Liber Abaci.

[2] Pacioli's work likewise teaches through examples, but it also develops arguments for the validity of its solutions through reference to general principles, axioms and logical proof.

This sort of appropriation has led some historians (notably including sixteenth-century biographer Giorgio Vasari) to accuse Pacioli of plagiarism in the Summa (and other works).

Many of the problems and techniques included in the book are quite directly taken from these earlier works, but the Summa generally adds original logical arguments to justify the validity of the methods.

[8] Its thoroughness and clarity (and the lack of any other similar work available in print) generated strong and steady sales to the European merchants who were the text's intended audience.

[8] Some of the same methods were described in other manuscripts predating the Summa (such as the 1458 Della mercatura e del mercante perfetto by Benedetto Cotrugli), but none was published before Pacioli's work, and none achieved the same wide influence.