Whenever there are runners on...
Each fielder may have different coverage or back-up responsibilities, even on something as simple as the throw back to the pitcher.

For instance...

LEFT FIELDER

L1 - Runner on third. Move to the line after every pitch, in case of catcher overthrows on pick-off attempt.
L2 - Runner at third, trailing runner at first. Steal of second is often countered by a decoy throw to short. If runner on 3 goes, break to the line.
L3 - Ball hit to right field. Back up the throw into second.

RIGHT FIELDER

R1 - Rundown. Back up second.
R2 - Ball hit to left field. Back up the throw into second.
R3 - Ball hit to leftside of infield or pitcher pick-off attempt. Move into foul territory in case of overthrow.

CENTER FIELDER

C2 - Runner at second, or runner at first. Be ready to back up pitcher's pick-off attempt, or catcher's overthrow.
C3 - Rundown. Move in quick to cover either a first-to-second or second-to-third rundown.

CATCHER

C1 - Bases empty, batted ball to right side. Run down the line parallel to runner, especially to cover possible overthrow from second baseman.
C4 - Even with a runner at first, catcher doesn't need to be glued to plate. Move up line to provide back-up at third.

PITCHER

P1 - Runner at one, ball hit to right field. Always back up third.
P2 - Runner at second, ball hit to any field. Back up home.
P3 - Any grounder or pop up to your left. Beat the runner to first.

In the middle of a developing play, you may also have responsibilities to back up plays at different bases.
Check out our interactive coverage clinic to get an idea of the possibilities.