Legibility is not a matter of taste or convention. It's a measurable property of how quickly and accurately a reader can distinguish letterforms and process text. Choosing a typeface for a brand involves three separate questions that get conflated constantly: does it read easily, does it carry the right tone, and can you legally use it commercially. Most font selection mistakes come from answering only the second question and assuming the other two are settled.
What the research actually says about serif vs. sans-serif
A common claim is that serif fonts are inherently more legible for body text, the little feet supposedly guide the eye along the line. This claim is older than digital typography and gets repeated constantly. The actual research doesn't support it as a general rule.
A controlled study by Arditi and Cho in 2005 used a specially designed parametric font with variants that did or didn't have serifs, and found no significant difference in reading speed between the two, either in rapid serial visual presentation or on paper. The visual feature that's supposed to be doing the legibility work didn't measurably change how fast people read.
What the research does show is more useful: legibility depends on a combination of factors working together, including typeface, spacing, and layout, not any single design feature in isolation. Poor contrast and overly dense line spacing increase cognitive load and reduce reading efficiency regardless of which typeface is chosen.
The rule
Legibility is controlled primarily by spacing, contrast, and sizing, not by the presence or absence of decorative strokes on the letterforms. A well-spaced, well-sized sans-serif will outperform a cramped, low-contrast serif every time, and vice versa.
What this looks like in practice
Where typeface choice does matter measurably: screen versus print context. Some typefaces, like Georgia, Verdana, Tahoma, and Trebuchet, were specifically commissioned for onscreen use, while many other commonly used fonts are digital adaptations of typefaces originally designed for print.
This is also where variable fonts have changed the practical picture. Since OpenType 1.8 introduced variable fonts in 2016, a single font file can contain a full range of weight, width, and other properties along continuous axes, rather than requiring separate files for each weight. For a brand system, this is a genuinely practical advantage, real versatility without the performance cost of multiple file downloads.
The part most people get wrong: licensing
Google Fonts are open source and free of cost, and they can be used commercially, including within a product that's sold commercially, with the specific terms governed by the license attached to each font. Google Fonts only includes fonts licensed under the SIL Open Font License.
What that license actually permits: the SIL Open Font License covers commercial use across client websites, paid apps, books for sale, merchandise, logos, and ad campaigns, with no fee and no permission required. Printing a Google Font on products you sell, t-shirts, mugs, posters, packaging, is fully permitted. Attribution is never legally required for end use, though it's appreciated.
The actual restriction, the one that trips people up: the only real limitation under the Open Font License is that you cannot sell the raw font files themselves as a standalone product. You can build a logo with it, print it on a sold product, embed it in client work, and modify it. You cannot package the font file itself and resell it as a font.
How to apply it
Test spacing and contrast before testing the typeface itself. A typeface that looks distinctive in a mockup can become illegible at body copy sizes if the spacing isn't dialed in.
Confirm screen readiness if the project is primarily digital. A typeface adapted from a print original isn't disqualifying, but check how it renders at small sizes on real screens first.
Use a variable font where weight versatility matters. A variable font family covers a full range, light captions through bold headlines, in one file.
Verify the license before presenting any typeface to a client as final. Confirm it's covered under the SIL Open Font License or an equivalent commercial use license before it goes into a brand guide.