Although usually translated into English as a seal ruler (印規 yingui) this tool has nothing to do with taking measurements. Seal rulers are usually in an “L” or “T” shape or as a flat disc with one quarter cut out. They can me made of just about any material; older ones are usually of wood or metal while modern ones are commonly made of plastic. They may be plain or decorated. For aesthetic reasons traditional materials are often preferred but, in the case of the seal ruler, clear plastic may be a better choice as you can see through the ruler and so more accurately position the seal where you want it on the page. See this separate post for use of the “T” shaped seal ruler.
When you apply your seal to an artwork, especially an important one, you want to be sure to get a good impression. Occasionally when you stamp your seal onto the art you get, instead, a very light impression (usually when your ink is dry). Without the use of a seal ruler it’s almost impossible (and not recommended to try) to re-stamp the seal in exactly the same spot. By lining the inside corner of a seal ruler up with where you want to place your seal on the art you can, if you don’t move the ruler, stamp it more than once.
Seal rulers are sometimes made from a curved section of bamboo. Although decorative they may not be practical since the raised middle of the curved section of bamboo is not in direct contact with the surface of the artwork. This means more chance of the seal shifting position the second time it’s stamped, defeating the purpose of the ruler.