The imperial Series

Further Information

Traditionally, a number of different sizes were defined for large sheets of paper, and paper sizes were defined by the sheet name and the number of times it had been folded. Thus a full sheet of 'royal' paper was 25 × 20 inches, and 'royal octavo' was this size folded three times, so as to make eight sheets, and was thus 10 ×  6 1/4 inches. Imperial sizes were used in the United Kingdom and its territories.

Source: Wikipedia