Tuffier's line is a straight line drawn between the iliac crests usually crosses the mid-line at the level of the spinous process of the L4 lumbar vertebra. The L3/L4 interspace can then be identified by palpation, and spinal anaesthesia performed at this level or below (Fig 1).

Anaesthetists often confuse the L3/L4 interspace for a higher one. To avoid damage to the spinal cord, spinal needles should always be inserted below L3.