"Large Bank Note" stamps refers to a major definitive series of postage stamps issued between 1870 and 1890. They are called "Bank Notes" because they were produced under contract by three private security printing firms—the National, Continental, and American Bank Note Companies—before the Bureau of Engraving and Printing took over production in 1894.

Key Characteristics
Size: They are called "Large" to distinguish them from the "Small Bank Notes" (or "Baby Bank Notes") issued from 1890 to 1894, which were reduced in size to cut production costs.
Paper and Grills: Early issues (National) often feature "grills" (embossed patterns intended to prevent reuse), while later issues moved from hard paper (National and Continental) to soft porous paper (American).
Secret Marks: To identify which company printed which stamp, "secret marks" (tiny design modifications) were often added to the original plates when contracts changed hands.

Historical Timeline
National Bank Note Co. (1870–1873): The original printers who introduced the designs.
Continental Bank Note Co. (1873–1879): Took over the contract and added secret marks to the 1¢ through 15¢ denominations.
American Bank Note Co. (1879–1890): After merging with Continental, they printed the same designs on soft porous paper and later introduced re-engraved versions with slightly modified details.