It combines head tube to the bottom bracket. It is the part that undergoes the strongest stresses, therefore it is characterized by different thicknesses and more abundant dimensions.
More thin in the central part and thicker at the ends. They are made in different ways depending on the manufacturers, but an elite section along the vertical axis gives a rigidity that can be excessive in the vibration distribution.
Sources:
http://www.cyclinside.it/web/category/componenti/
http://www.albanesi.it/corsa/ciclismo.htm