Why is the nib of a fountain pen split?

A fountain pen allows a controlled flow of ink because of its specially designed flow system for ink and air. The flow system consists of an ink reservoir, a feed bar (also called feed) and a metal nib. 

The nib is split starting from a breather hole till the tip to form two tines (branches). The feed has a longitudinal flow channel on its top surface starting from the inside end almost up to the forward end and it is covered by the nib. The channel is a broad groove and along its floor are cut two or more narrower grooves (called fissures). The broad channel brings ink up to the breather hole and the split of the nib. The split carries the ink to the tip and paper. The channel allows an occasional flow of air in the opposite direction into the reservoir so that pressure there does not decrease too much due to emptying of the ink. When such an air-flow takes place the fissures still contain ink and enable immediate filling of the channel. The nib tip has a special shape consisting of two hemispherical ends at the bottom of the tines. Ink forms a meniscus between these two rounded parts and spreads on paper as soon as a contact is established. The split width will change if pressure is applied and this will change the flow. The feed often has slots, combs or other design elements so that the capillary action can draw ink and hold it to have a steady flow or to avoid an overflow. Even when a pen is not used for a while, capillarity replenishes through the split, the evaporated ink and the pen is kept ready. The capillary force that maintains a continuous ink flow also helps to support ink from dripping due to gravity while not writing.

Source: thehindu.com