Simplicity is the ultimate sophistication.
– Leonardo da Vinci

Keep It Simple Stupid (KISS).

It isn’t hard to make things difficult, anybody can do that. Even someone stupid. The skill and art, is in making it simple.

Simple is: the essence; the most important part; something that is repeatable; provides the same outcome; that if you remember anything, remember this;

The KISS principle was first adopted by the US Navy¹, for the design of their naval ships. With the premise being, the more complex a ship is the more likely it is to breakdown and the harder it will be to maintain.

Keep it simple, when you get too complex you forget the obvious.
— Al McGuire

It is not only true for ships, but any process, system or design. By keeping your work process simple, you are heading towards an error-free work environment. Meaning there is a decreased chance of a safety, quality or environmental incident. And if there is a failure in the process, it is less likely to have a severe consequence.

Keep your workflow simple, there is less chance of things going wrong.

Life’s like art. You have to work hard to keep it simple and still have meaning.
– Charles de Lint

As a guiding principle, design should aim to avoid complexity. The most important part of keeping a design simple, is being consistent within the design. The more variants in a solution, the more complex a design gets.

Aim to provide a solution that can be utilised in as many scenarios as possible.

If you can’t explain it simply, you don’t understand it well enough.
— Albert Einstein

You never really understand something, until you have to teach it to someone. Because it forces you to articulate a concept. It has the side-effect of reducing the concept to its most simple key elements in a logical order. So that the majority of people can understand what you are explaining.

If people don’t understand you, the chances are you haven’t simplified the concept enough.

It seems that perfection is reached not when there is nothing left to add, but when there is nothing left to take away.
Antoine de Saint Exupéry

The principle is also true of writing. The art to a good article is being able to express yourself in as few words as possible. While not losing the essences of your message.

Check out the Engineering Rule Book for the other rules.