Consistency is not a Holy Grail of design. Consistency is a characteristic of a UI that needs balance. Too much of it, and too little are both terrible.
Consistency is a characteristic of a UI that needs balance. Too much of it, and too little are both terrible.
Consistency is essential between elements that: are the same, work the same, or result in the same outcome.
Consistency for the sake of reusing something already done is a simplification that can result in confusion or missed opportunity for improvement.
Do you want all your books to be the same?
Do you want all the controls in your car to be the same? To get rid of dials, buttons, levers, switches, crowns? It would be very consistent, but also very hard to use.
Consistency is also crucial in narrow scopes, locally, but rarely spans many dimensions.
For example, we might want all the primary buttons to look the same, but we do not want all of them saying OK. We want to use sentence case and capital case consistently. Consistency here is limited to how it looks. Sometimes that needs to change – destructive actions require a different colour of a button, and maybe a confirmation – they diverge from consistency.
Imagine two elements, and how they compare based on how they look (form), what they do (function), and what is a result (outcome):
|✻||✻||✻||Seems consistent, and should be this way|
|–||✻||✻||Maybe the form is different due to context? Like big and small size of a button?|
|✻||–||✻||(TBH, I need to think more about this one.)|
|✻||✻||–||People might expect the same thing, so maybe consistency is not a good thing here?|
|–||–||✻||You might want to think about adding some consistency!|
|✻||–||–||Definitely not a good idea for things to look the same if they do and work differently. Consistency might not be a good thing|
|–||✻||–||Maybe adding consistency can be an improvement?|
|–||–||–||Avoid making the elements consistent!|
↑ The two scenarios when a function is different from the rest are the hardest.