The midclavicular line is one of the surface reference lines used in surface anatomy of the thorax.

It is a parasagittal vertical plane that passes halfway through the body of the clavicle. The lateral portion of the clavicle is close to the highest point of the shoulder joint (the acromioclavicular joint), while the head (medial aspect) of the clavicle is found just lateral to the jugular notch of the sternum, a depression in the superior aspect of the sternal manubrium.

Although it was originally used as a thoracic reference point, the continuation of the midclavicular line into the abdomen is used today as a reference for laparoscopic procedures, to indicate the location where a trocar must be introduced into the abdomen. As an example, in a laparoscopic cholecystectomy, one of the four trocars (in this case a 5mm trocar) is introduced into the abdomen on the right midclavicular line four fingerbreadths inferior to the costal margin (the lower border of the ribs).  This trocar is used to manipulate the gallbladder and is placed in the gallbladder infundibulum.

It is also one of the lines used to describe the abdominal regions.

Sources:

Sources:
1. "Clinical Anatomy" Brantigan, OC 1963 McGraw Hill
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. Davis, Gwilym G. "Applied Anatomy: The Construction of the Human Body Considered in Relation to Its Functions, Diseases, and Injuries"; Philadelphia: J.B. Lippincott Co., 1910
Image modified from the original Davis, 1910