The sternal horn is flat against the second rib cartilage. The sternal angle is where the sternal stalk and the sternal body meet; it protrudes forward, can be seen and felt on the body surface, and is often used as a marker to determine the second rib. Its significance is that if there is a spontaneous pneumothorax, the sternal angle can be used to select the intersection of the second intercostal space and the midclavicular line as the puncture site for thoracentesis to treat the pneumothorax. In addition, the sternal angle can also be used to determine where the second intercostal space is, which can be used to select the site for surgery and then draw the surgical marker line.