 PLCs, or Programmable Logic Controllers, are used in complex automation systems to carry out a series of process tasks. LadderLogic is used to program these PLCs in order for them to communicate a specific sequence to each part of the system. LadderLogic was originally a written method to document the design and construction of relay racks. Each device in the rack would be represented by a symbol on the ladder diagram along with the other external items in the system like pumps and chillers. These diagrams were used so relay rack setups could be replicated at a later date or in another location. As technology advanced, traditional relay racks gave way to the more efficient and expandable PLC, and old ladder diagrams evolved into the programming language that we know today as LadderLogic. The motivation behind utilizing the ladder diagram as the base for the new programming language was pragmatic. Since it was using something that technicians were already familiar with, it did not require intensive training in complex computer programming languages like Fortran. LadderLogic notation is best suited for systems where binary values are required. For example, a contact is either open or closed. Here is a sample of a very simple LadderLogic program. Here we see a LadderLogic program with two regular contacts represented by open and closed brackets that are called keyswitch1 and keyswitch2. And one regular coil represented by open and closed parentheses that represents a door motor. A program of this nature could be utilized for physical security within a system. Since keyswitch1 and keyswitch2 are aligned in sequence, both of these conditions must be true in order to engage the door motor. So both switches must make contact in order for the door to open in this scenario. LadderLogic has an interesting past and clearly illustrates how necessity is the mother of invention. LadderLogic programs in PLCs can be very powerful and have the ability to control complex industrial processes.