Lumbar triangle

The lumbar triangle can refer to either the inferior lumbar (Petit) triangle, which lies superficially, and the superior lumbar (Grynfeltt) triangle, which is deep and superior to the inferior triangle. Of the two, the superior triangle is the more consistently found in cadavers, and is more commonly the site of herniation; however, the inferior lumbar triangle is often simply called the lumbar triangle, perhaps owing to its more superficial location and ease in demonstration.

Inferior lumbar (Petit) triangle
The inferior lumbar (Petit) triangle is formed medially by the latissimus dorsi muscle; laterally by the external abdominal oblique muscle; and inferiorly by the iliac crest. The floor of the inferior lumbar triangle is the internal abdominal oblique muscle.

Superior lumbar (Grynfeltt) triangle
The superior lumbar (Grynfeltt) triangle is formed medially by the quadratus lumborum muscle, laterally by the internal abdominal oblique muscle, and superiorly by the 12th rib. The floor of the superior lumbar triangle is the transversalis fascia and its roof is the external abdominal oblique muscle.