Low midline sill sign of a patient with lumbar spondylolisthesis. Inspection of the low back to detect low midline sill sign. In this case, lumbar lordosis increases and a sill like a capital “L” is observed at the L4-5 level. The skin around the sill is wrinkled and thick compared with surrounding skin (A). Palpation of the low back to detect low midline sill sign. The examiner palpates the interspinous space and evaluates the position of the upper spinous process in relation to the lower spinous process (B).