Each lung has three borders:

Inferior border - separates the diaphragmatic surface from the costal surface and medial surface; it is thin and sharper at the site where it separates the diaphragmatic surface from the costal surface but more rounded at the site where it separates the diaphragmatic surface from the medial surface; it is also known as the diaphragmatic border;
Anterior border - it is relatively thin and sharp; separates the costal surface from the medial surface and corresponds to the sternal line of the pleural reflection (lines of the pleural reflection mark the areas where the parietal pleura changes its direction from one lung surface to another); it is also known as the sternal border;
Posterior border - it is thick, and it also separates the costal surface from the medial surface (their posterior aspects).

The anterior border contains two landmarks - the cardiac notch and lingula. The cardiac notch is a deep notch created by the apex of the heart and located on the lower aspect of the left anterior border. Inferior to the cardiac notch is the lingula - a tongue-like projection of the superior lobe of the left lung.

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Borders of lungs