They join together at your SI joint to form your pelvic girdle or hips. If you look at someone's lower back, you can see the two sacroiliac joints as dimples on either side of their spine ...

“To paraphrase Joseph Pilates: ‘You are as young as your spinal column – a stiff ... at 90° Inhale to move legs and pelvis as one unit to one side Exhale to return to the start position ...