ClipArt ETC
"Holding the pencil in a plane perpendicular to the line of sight, marking with the thumb the length of pencil which covers a line of the model rotating the arm, with the thumb held in position, until the pencil coincides with another line, and estimating the proportion of this measurement to the second line." —French, 1911

Estimating Lines using Pencil

"Holding the pencil in a plane perpendicular to the line of sight, marking with the thumb the length…