Traditional Stationery Paper Sizes

Traditional paper sizes for letter-writing, before everyone just started using 8.5 x 11 "Letter" for everything, all the time.

Writing Sheet Sizes

• Monarch: 7.25" x 10.5" - Traditional (men's) personal stationery, sometimes called "Executive" (wrong).
• Executive: 7.5" x 10" - Seems to be more popular now than actual Monarch. Not sure how/why it evolved, but that seems to be the proper size back when it was commonly available. (The slight size difference from Monarch might have been part of the size rationalization that tried to make more sizes of sheet available from the same rolls without waste.)
• Tabloid/Ledger: 11" x 17" - "Ledger" and "Tabloid" are the same size sheet, but Ledger is landscape, Tabloid is portrait. (Not an important distinction when buying loose sheets, but pretty important if you're buying a bound book of some sort.)
• Sovereign: 8.5" x 11" - The predecessor to "US Letter"; noted in some early 20th c. sources that it's the proper choice for "business correspondence", rather than friendly or personal letters.

Envelopes

Each size originally had a matching envelope size.

• Monarch Envelope: 7.5" W x 3.875" H

Sources

Monarch sizes are used largely by executives and professional men; for letters of a semi-social and commercial character. Sovereign sizes are employed more especially for commercial correspondence that partakes somewhat of a personal nature.

Handbook of Quality Standard Papers, 1922.