The Obliques

The external and internal obliques are part of the core muscles. They are located on each side of the torso, and the external oblique is superficial to the internal oblique. The external oblique is a relatively large muscle extending from the lower half of the ribs around and down to the pelvis. At the bottom part, it connects to the iliac crest, pubis and linea alba (a connective tissue made of fibres that runs vertically along the inside of the abdominal wall). The internal oblique runs from the lumbar fascia to the inguinal ligament and iliac crest, and the lines alba to the bottom edge of the ribcage.
These muscles assist in spinal rotation, side bending, and breathing as they pull the chest down during exhalation.
Not only to get a fine waistline, but the obliques are also important muscles for posture and breathing.