With the patient in the lateral or sitting position, use the index and middle fingers on either side to palpate the tops of the iliac crests. Once identified, bring the thumbs together into the midline and feel for the intervertebral space - this is L4/L5.

The spinal cord ends at L1/L2, so the L3/L4, L4/L5 or L5/S1 interspaces can be used for lumbar puncture.