In his mind, Tammet says, each positive integer up to 10,000 has its own unique shape, colour, texture and feel. He has described his visual image of 289 as particularly ugly, 333 as particularly attractive, and pi, though not an integer, as beautiful. The number 6 apparently has no distinct image yet what he describes as an almost small nothingness, opposite to the number 9, which he says is large, towering, and quite intimidating. He describes the number 117 as "a handsome number. It's tall, it's a lanky number, a little bit wobbly." He described David Letterman with the number 117 in these terms when interviewed on the Letterman Show. In his memoir, he describes undergoing a synaesthetic and emotional response for numbers and words.