The formal lines of authority from highest to lowest ranks are known as scalar chain. According to Fayol Organisation should have a chain of authority and communication that runs from top to bottom and should be followed by managers and subordinates.
In the diagram, O is heading 2 teams where A and X are at the same level, B and Y are at the same level and C and Z are at the same level. The line of communication is clearly defined A to B to C and X to Y to Z Authority responsibility relationship is also very clear. But C and Z cannot directly contact each other. This sometimes may take lot of time during an emergency to communicate as per the prescribed flow to avoid this problem a system known as 'Gang Plank' was introduced where in C or Z could contact each other and same way B and Y could contact each other.
