The Japanese title of this tool is Tombo; in English it means Dragonfly. The dragonfly’s history is confined to Japan, where it is used to measure the diameter and depth of bowls, cups, and plates in order to produce a series of vessels of the same basic dimensions. It can be considered as an adjustable template.
Japanese potters choose this tool out of the many other measuring tools in use today for its lightness, affordability, adjustability, ease of use and charm. You can set a dragonfly for the measurements of one pot, then put it aside, and then use again whenever need. It is common for potters to have several preset dragonflies sitting by their wheel for use as needed.