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This 20-sided die is numbered in the circular Gallifreyan writing system used in Dr. Who. Many guides are available online for translation, in case your GM doesn't want to take your word for it. Each ring outside the innermost ring represents a digit, and the number of lines inside that ring determines that digit's value (a tiny circle/dot=5 lines).
Check out the 20mm version in my shop for a smaller and more affordable print!