Sloppy and inconsistent usage of font sizes, weights, and colors are a telltale sign of someone who doesn’t quite know what they are doing with typography. This is about interface design, where there needs to be a reliable and consistent system for a user to learn and understand. With the rule of four, you can start developing your type system; however, as with everything in design, a law is meant to be broken. Once you are aware of the boundaries and why they exist only. This rule will help you remember some essential ingredients of typography for interfaces.

In the example above, as of iOS 14 the type sizes would be as follows:

• Titles & Actions = 17px
• Subtitles & Body Copy = 15px
• Secondary Actions = 13px
• Metadata = 11px

On nearly every single Apple-designed interface screen, you can find some combination of these sizes. Within the latest iOS updates, you can find some titles temporarily set to 34pt, doubling the standard size of 17pt, but only temporarily. Once it scrolls out of position, the trusty 17pt titles reveal themselves.