Shear reinforcement shall be provided in any of the following forms:
a) Vertical stirrups
b) The bent-up bar along with stirrups 
c) Inclined stirrups Shear reinforcement shall be provided to carry a shear force equal to
Note that, the shear reinforcement increases as the spacing among the stirrups decreases and vice versa. 

As shear strength requirement is more at the supports than the center, the spacing of stirrups increases towards the center of the beam.