The coil undergoing commutation and the coil getting short circuited are two different coils in a practical DC machine. These coils have different positions relative to the armature core and the commutator segments.

The coil undergoing commutation is the coil that is connected to the commutator segments at the moment when the brushes make contact with the segments. This coil is in the process of transferring the current from the armature winding to the external circuit. During this process, the coil experiences a change in the direction of current flow, which is known as commutation.

On the other hand, the coil getting short circuited is the coil that is connected to the brushes at the moment when the brushes lose contact with the commutator segments. This coil is short circuited and does not contribute to the output of the machine.

To summarize, in a practical DC machine:
- The coil undergoing commutation lies along the commutator segments.
- The coil getting short circuited lies along the brushes.

These positions are crucial for the proper operation of the machine and to ensure smooth commutation. The coil undergoing commutation must be in contact with the commutator segments to transfer the current to the external circuit effectively. The coil getting short circuited is disconnected from the external circuit to prevent any short circuiting and to allow the commutation process to occur smoothly.

Proper commutation is essential for the efficient operation of a DC machine. It ensures that the current in the armature winding remains unidirectional and the machine generates a steady output. The coil undergoing commutation and the coil getting short circuited play crucial roles in achieving this proper commutation.