From Marguerite Ogle,
Your Guide to Pilates.
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Definition: The bony parts that you feel under you when you sit up straight on a firm surface.

They are part of the pelvic girdle and technically known as the Ischial Tuberosity.

Also Known As: Sits Bones, Sitz Bones, Ischial Tuberosity

Alternate Spellings: sits bones, sitz bones

Examples: Many Pilates exercises, such as spine stretch and saw, begin with sitting up straight on the sit bones.