What is a No. 1 pencil?

A former grad-school professor of mine enjoyed touting the benefits of No. 1 pencils. We had never heard of such a thing. Apparently, not many people have, especially in the age of standardized testing the ubiquity of the No. 2 pencil. My professor believed the pencils to be superior for writing in books because the marks could be more easily erased, as the lead was softer. Years later, I bought a box of No. 1 pencils and found they weren't that easy to locate as an item. (This must have been before Amazon started carrying everything.) When I did find a box, I was disappointed to learn that sometimes in the era of modern pencils the "No. 1" designation was just a marketing gimmick and bore no relation to the softness of the lead. I think I will buy another box now (on Amazon) to test this theory out in a more empirical way. Surely the graphic-arts industry demands a wide variety of pencils, whatever the numbering system.

Photo by Hafiz Issadeen. (Used under Creative Commons license. Image is unaltered.)