A set of 3 rulers.
For ruling repetitive guidelines of 4mm, 5mm,  6mm quickly in 1:1, 2:1 and 3:2 ratio.
Can draw slant line of 30°, 45°, 52°, 55°, 60°, 85° for different scripts including Copperplate, Spencerian, Gothic, Italic and Uncial etc.
Center Envelope Guidelines for A, C, and DL Types.
 
Other rulers from 21Lines: 
A5 Line Repeater here
Lettering Grid Ruler here