A System is improved when similar parts are expressed in a similar way.
The principle of consistency refers to designing systems so that similar parts are recognisable making them easier to learn and more usable. Consistency enables people to transfer knowledge to new situations, learn quicker and focus attention on important aspects of a task. There are four (4) types of consistency. Aesthetic, Function, Internal and External.
Aesthetic consistency refers to consistency of style and appearance. An example of this would be a company logo that uses a consistent font, colour and graphic. The use of aesthetic consistency enhances recognition. For example, car brands are recognisable due to the practice of placing the company logo at the front of the vehicle on the grill or the bonnet. More prestigious car brands such as Rolls Royce or Ferrari are examples of this where the logo has become associated with quality, prestige and success.
Functional consistency refers to consistency of meanings and actions. For example, traffic lights are consistent across states and nations. Red signals stop and green signals go. Functional consistency improves usability and learnability by using peoples existing knowledge about a system. For example, electronic equipment will often have a green LED indicating the device is turned on, media recorders or playback devices such as DVD or Blu-Ray players will use symbols for Play, and Stop which are consistent across all devices. By using consistent symbols for these devices, users can use the device using existing knowledge which they may have learnt from other similar devices (such as a VHS player).
Internal consistency refers to consistency with other elements in a system. For example, signage at a sporting stadium are consistent with each other. This type of consistency provides familiarity and trust for people.
External consistency is referring to consistency with other elements outside of one organisation such as a phone busy signal or an emergency siren from Police, Fire or Ambulance. This is often difficult to achieve as different countries and organisations often do not design to a common standard. For example, whilst emergency sirens may be consistent in Australia, emergency sirens in other parts of the world will often sound different.