When we are creating systems or products, we try to meet the needs of a wide range of users of our systems or products. Say we are designing a kitchen for a small family apartment. Different people have different expectations from the kitchen. There are many kind of refrigerators and microwaves and dishwashers available to people. So if the builder of the house created a kitchen along with the fridge and dishwasher, many families may like the rest of kitchen but may not like the fridge or the dishwasher. So it is better to design the kitchen in such a way that the residents can use their own fridge. The builder of the house should leave out space for the fridge, keep electrical wiring, and ventilation etc, so fridge gets the space, electricity, air circulation etc. But for this to work the fridge also has to meet some guidelines. Someone designing a kitchen for a small family apartment can not give so much flexibility that someone can fit industrial sized walk in cold storage in it for example. The fridges that are designed to be used in kitchen of small family residential apartments should all have some common characterestics as well. This way of designing is called modular designs. We find examples of modular design everywhere. The airport carry on compartmnets storage are modularly designed for carry on bags that are designed for those storage, they all have dimensions that fit the airport compartment. Electrical sockets are modular to fit electrical plugs. Books are designed to fit in bookshelf, and bookshelf are designed for books. Lego blocks are modular. Each modular design has a `module specification`, and two counter parts, `module provider` and the `module consumer`. In the case of the kitchen and fridge, the `module specification`, like the height, width, depth, max weight of the fridge, the power rating of power outlet, the voltage, the alternating current frequency, the amount of air circulation (which limits how much heat can the fridge produce during operation, without heating up the kitchen itself, because that is how fridge works, it keeps the content cool by transferring the heat from inside to the outside, and in the process producing some excess heat, determined by the carnot cycle effeciency of the compressor in the fridge). Then we have `module providers` like various manufacturers who sell fridges like LG, Samsung, etc. And the `module consumer` is the kitchen which is being designed. Once a specification is created, both producer and consumer of the module must follow the specification, so if specification says fridge must have a maximum height of 3M, the builder of small family apartment (the consumer) must ensure there is at least 3M of height left else the people living in the house will not be able to bring in their fridge, which may be 3M tall. And similarly the electrical appliances company must ensure the fridge has a maximum height of 3M, else the fridge you buy in the store will not fit in the kitchen.