The first two posterior intercostal aeries are branches of the highest (superior) intercostal aery of the costocervical trunk; the remaining nine branches are from the thoracic aoa. The internal thoracic aery gives off the upper six anterior intercostal aeries and is divided into the superior epigastric and musculophrenic aeries, which gives off anterior intercostal aeries in the 7th, 8th, and 9th intercostal spaces and ends in the 10th intercostal space where it anastomoses with the deep circumflex iliac aery. So answer to this question is Lower six posterior because they are arising from lower aoa directly.