Modular Scale
A modular scale is a list of values that share the same relationship. These values are often used to size type and create a sense of harmony in a design. Proportions within modular scales are all around us from the spacing of the joints on our fingers to branches on trees. These natural proportions have been used since the time of the ancient Greeks in architecture and design and can be a tremendously helpful tool to leverage for web designers.
Ems work especially well with modular scales as their recursive properties mimic modular scales making them more predictable and easier to manage. Pixels and other units work just fine and breakpoints in responsive web design can naturally fall on your scale to create better relevance to your text as all the values in your layout harmonize with each other.