Sarcomere is the area between two:

Differences in the refractive indexes of the various pas of the muscle fiber are responsible for the characteristic cross-striations seen in skeletal muscle when viewed under the microscope. The pas of the cross-striations are frequently identified by letters. The light I band is divided by the dark Z line, and the dark A band has the lighter H band in its center. A transverse M line is seen in the middle of the H band, and this line plus the narrow light areas on either side of it are sometimes called the pseudo-H zone. The area between two adjacent Z lines is called a sarcomere. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 5.