D3

The D3 format prescribed in the German paper size standard DIN 476 measures 272 mm x 385 mm, giving it a landscape orientation with dimensions roughly between ISO A4 and A3. With an area of 0.105 square meters, the proportions with a width-to-height ratio of 1.25 suit technical diagrams, graphs, ledger tables, and similar spreadsheets requiring a greater horizontal expanse. In particular, the aviation industry made use of D3's dimensions for flight books and aircraft operating handbooks. The size also nearly matches the "Foolscap Folio" paper popular in the UK.

As the smallest format oriented in landscape rather than portrait in the German system, D3 likely emerged from existing transitional sizes in early 20th century technical documentation. It's just a bit more elongated than the ISO 216 A series. As the DIN standard intended to consolidate paper sizes, the landscape mode and sizing incremental to A4/A3 would enable flexibility preferred by some industries like aeronautics. However, just as C3 and B3 never gained adoption over A3, the D3 format saw declining favor relative to standardized dimensions. Nonetheless, it had its applications before proliferation of A4 and the international standards.