Here is a principle with many applications in graphic design: elements should either coincide or contrast. It is surprising how often this golden rule of design will answer a wide range of questions—choice of typography, layout, image style and color. We will look only at typography and layout here.
One of the easiest applications of Coincide or Contrast is in typography. It is very difficult to use two different sans-serif fonts together. There is not enough difference to provide meaningful contrast but too much difference to coincide.
Georgia and Berhard Modern Std (same point size)
The same is true about trying to use two different sans serif fonts.
Helvetica Neue LT Std. Light and Gill Sans Light (same point size).
Perhaps an example in a totally different realm might illustrate. It’s like wearing stripes and plaid together; it just doesn’t work. Stripes with a plain fabric would give a nice contrast without visual violence. Plaid with a plain fabric could also work.
The same is true when using serif fonts: two different serif fonts are unlikely to work well together. They have too much difference to coincide, but not enough to contrast. They look mismatched because they are. They clash.
The same idea applies to large font families that contain condensed and extended variants. For instance, we should consider Helvetica Condensed or Helvetica Extended to be different typeface designs from Helvetica, because they are different font designs. The fundamental shapes of the letterforms have been changed. They can no longer coincide.
This does not apply to italic or boldface, which are weight or emphasis alternatives within a single font. They were created to be used together.
A better alternative is to use either all the same font or a combination of one serif font with one sans-serif font. Then there is enough contrast.
Within a layout, one single font can work very well, with only weight and size to distinguish headlines, subheads and text. Captions are usually offset from body text with size and/or weight variations to differentiate.
Coincide or Contrast in a Layout
Positioning elements is another way to apply the Coincide or Contrast principle. Unless there is a specific reason to do otherwise, aligning your elements will place the focus on the content and not the arrangement, which is as it should be. The purpose of design is to aid communication, not draw undue attention to itself. If elements are only slightly misaligned, it just looks like a mistake.
Adapted from Logo Design Theory: How Branding Design Really Works