This beautifully made ruler was used to make perfect parallel lines. One ruler made of mahogany rotates and the other  made of ebony doesnt.
APPLETON'S PATENT DOUBLE RULER. For marine navigation.
With this Ruler perfectly parallel lines are produced, by drawing the Pen along the Black Ruler and rotating the Light one in the usual way. The Black Ruler because it doesntrevolve, means that should Ink adhere to it-it does not come in contact with the Book or Paper, thus preventing blots or smudges, and the necessity of constant wiping.