Before the twentieth century, paper was made in a mold. The size was set by a frame; at the bottom was a wire mesh. Workers would dip the mold into a vat of pulp — usually made from rags, not wood pulp — and then shake the mold to spread the pulp across the bottom. The wire mesh would leave an impression on the paper, visible when you hold it up to the light. The horizontal lines are closely spaced, and are called “wire lines"; the vertical lines, thicker and more widely spaced, run from top to bottom.
Most paper today is made without wire and chain lines, although some fancier papers — known as “laid” paper -- have the wire and chain lines artificially added to give it an older feel.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.