Gbex says: The rectus abdominis muscle is a paired muscle running vertically on each side of the anterior wall of the human abdomen. There are two parallel muscles, separated by a midline band of connective tissue called the linea alba (white line).
It extends from the pubic symphysis/pubic crest inferiorly to the xiphisternum/xiphoid process and lower costal cartilages (5–7) superiorly.
It is contained in the rectus sheath.
The rectus is usually crossed by three fibrous bands linked by the tendinous inscriptions.
While the "sixpack" is by far the most common configuration of the muscle bellies of the rectus, there exist rare anatomic variations which result in the appearance of eight ("eightpack"), ten, or—even rarer—asymmetrically arranged segments.