The Stipples, also known as the red-hot stipples, are a type of punishment that is employed by magicians on misbehaving demons to keep them in line. This punishment is known to be very painful, and Bartimaeus has been subjected to it, likely more than once.
Nathaniel once threatened to use this punishment on Ascobol, asking if he had a preference betwen it and the Unfortunate Hug. Ascobol replied that he could not be so cruel and sought to deflect blame onto Bartimaeus. Growing bored of his protestations, Nathaniel dismissed him.