Comment by Brajeshwar

Not all "sans-serifs" are similar, i.e. they cannot be put in a single class opposed to serifs.

The sans-serifs designed before WWII have evolved towards a maximum simplification of the letter forms, eliminating not only the serifs, but also the contrast between thin lines and thick lines and also some other details of the shapes that differentiate the letters. The extreme simplification was achieved in some of the "geometric" sans-serifs, where most of the letter shapes were simplified into circles and straight lines. The motivation for shape simplification was that in the beginning the sans serifs were intended for advertising, i.e. for printing on cheap paper with cheap devices, which could not reproduce fine details. More over such texts were intended to be readable from great distances, from where fine details cannot be perceived.

After WWII, some sans-serifs have just reproduced with negligible changes some of the earlier sans-serifs, e.g. Helvetica and the like.

Most of these sans-serifs, like Arial, are extremely bad for use in computers, because they have a lot of ambiguous letters and digits. That mattered less for English texts, which have high redundancy, allowing the guessing of the intended letters, but in computers there are a lot of abbreviations, keywords, identifiers, expressions and other kinds of character strings with low redundancy, where it may be difficult or impossible to recognize the intended letters or digits.

However, there are also several kinds of very different sans-serifs among those designed after WWII, which attempt to remove the disadvantages of the classic sans-serifs.

One kind is the sans-serifs that have only the goal to remove all ambiguities in letters, digits and other symbols, like most of the monospace typefaces intended for programming.

Another kind, besides removing ambiguity also reintroduces some of the letter shapes from serifs, which have been simplified in the classic sans-serifs, e.g. double-storey "g" and "a", old-style digits, distinct shapes for the italic variant of a font, etc. These slightly more complex shapes increase the distinctness of the letters and digits, making them more easily recognizable. An example of such a typeface is FF Meta.

Finally, there is another kind of sans-serifs that also reintroduces the contrast between thin lines and thick lines. Moreover, among these there are typefaces, e.g. Optima or Palatino Sans, which (on displays with high enough resolution) produce an optical effect similar to serifs, which is achieved not with serifs but by replacing the straight side edges of the lines with slightly concave edges.

In my opinion these kinds of sans-serifs that are intermediate between classic serifs and classic sans-serifs are superior to both classic types, especially for computer use.

Despite being less affected by low screen resolutions, the classic sans-serifs like Helvetica, Arial and many others should never be used for anything, because of their ambiguous glyph shapes.

Serifs have non-ambiguous glyph shapes, but they are heavily distorted at low-screen resolutions or when seen from a distance, so they are also not recommended in many applications.

The good choice is in most cases to use one of the sans-serifs with non-ambiguous glyphs, or for a more beautiful text one of the typefaces inspired by FF Meta or one of those inspired by Optima. For example, my default typeface is Palatino Sans.

Any study about the legibility of serif and sans-serif typefaces that does not include a separate category for non-classic sans serifs is deeply flawed and its results are meaningless.