Why do we use 8 1/2” X 11” paper?

In the early part of the 20th Century before WWI, many industrialized countries started discussing the advantages of standardizing paper sizes to make it easy for their post offices to handle mail. Most accepted the ISA standard sizes in 1925.

The USA held a joint meeting between manufacturers, distributors, and users and drafted the Simplified Practice Recommendation R22. This formalized the existing sizes most frequently used and did not add new ones. It became effective June 15, 1933.

Why the sizes were frequently used is an interesting question. One answer is the Dutch invented the two-sheet mold in 1690. The average maximum stretch of an experienced vatman's arms was 44". Many molds at that time were around 17" front to back because the laid lines and watermarks had to run from left to right. This limited the size of the two sheets.

In the nineteenth, century they became one of the ways handmade papermakers could survive competition from machines.

Article by Institute of Paper Science & Technology