The receiving partner lies on their back with knees up and legs apart. The penetrating partner lies on their side perpendicular to the receiver, with the penetrating partner's hips under the arch formed by receiver's legs. This position is sometimes called the T-square.
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A T-square is a technical drawing instrument used by draftsmen primarily as a guide for drawing horizontal lines on a drafting table. It may also guide a set square to draw vertical or diagonal lines. Its name comes from its resemblance to the letter T.
Learn more about T-squares from Wikipedia.