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Anatomically, two triangular shaped spaces can be identified: the triangle of auscultation and the lumbar triangle. The triangle of auscultation is bordered by the trapezius muscle, latissimus dorsi muscle, and the medial border of the scapula. The lumbar triangle is bordered by the latissimus dorsi muscle, external abdominal oblique muscle, and iliac crest.

Links and References:
Grant's: 4.47
Netter:160 and 163
Rohen/Yokochi: 209