It is most common in the abdominal aorta, where it is known as abdominal aortic aneurysm (triple AAA). If it occurs in the thoracic aorta, it is known as thoracic aortic aneurysm. A true aneurysm involves all three layers of the blood vessel wall. A false aneurysm of the artery is contained only by the two outer layers of the blood vessel wall and clot.