Where did the 1 1/4-inch standard size for eyepiece barrels come from?
This barrel size was adopted shortly before 1890 by John A. Brashear in Pittsburgh, Pennsylvania, says Bart Fried of the Antique Telescope Society. Then W. & D. Mogey of Plainfield, New Jersey, and other firms followed suit. By choosing 1 1/4 inches, they were breaking with the traditional 1 1/8-inch barrels used by the older and more prestigious firm of Alvan Clark & Sons in Cambridge, Massachusetts.
Today, 1 1/4-inch (31.8-millimeter) barrels are made and sold worldwide. They have largely supplanted the 0.965-inch (24.5-mm) barrels long in vogue with such firms as Zeiss and Unitron. But the advent of large, wide-field eyepiece designs has made the 2-inch barrel a popular alternative in recent years.
— Roger W. Sinnott