Why Text in All Caps is Hard for Users to Read

This is an excellent overview of the evidence for the various models of reading. It is far better than the parent article. Submit it on its own!

It dates to the medieval copyists---mostly monks---who developed new ways of writing that were faster. In some cases the letters connected (what we'd call cursive), sometimes just flowed (italic), sometimes they were just shorter and rounder (uncial/half-uncial). The dominant form was this new style. BUT, for the beginnings of chapters and/or pages, they would revert to the old Latin letters, often with a great deal of decoration. Thus the capitals remained in active use even as the running text evolved. The printing press narrowed the number of distinct styles somewhat, but even today if you look at handwriting across Europe, the capitals are pretty recognisable and would look familiar to an ancient Roman, but the rest of the text diverges a lot.

...which doesn't exactly answer your question of what it's "for", I guess. But to some extent, we have two cases because that's what history gave us, and now we make use of them as best we can.

I think the existence of capitals has more to do with history and less with utility. As you said, it’s not obvious why you would need capitals.

The Romans started with nothing but what we today know as capitals [1] (imported from elsewhere but that is another story). Everyone who needs to write a lot and fast (e.g. merchants) will optimize the letters for this purpose and thus dramatically change how the letters look. That’s how the Romans got their cursive [2]. If, on the other hand, your task is to chisel letters into stone or copy a book you can be extra diligent, an opportunity to clean up letterforms [3].

Those two writing styles – cursive and capitals – were not intended to be combined with each other. It’s only later, in medieval times, that both letterforms are used together [4].

[1] Example: http://en.wikipedia.org/wiki/File:Hermes_16_illustration1.pn...

[2] http://en.wikipedia.org/wiki/Roman_cursive

[3] Square Capitals, example: http://en.wikipedia.org/wiki/File:Arch.of.Titus-Inscription.... – Uncial: http://en.wikipedia.org/wiki/Uncial_script

[4] Example: http://en.wikipedia.org/wiki/File:Minuscule_caroline.jpg

In this context, the German use of caps is interesting. They capitalize every noun, and I believe that such a use has been proven to allow for speedier reading than the capital use in other Western languages (i.e. English).

Hindi doesn't have capital letters either. I think we recognize a name more by it's context than the case of the first letter. If I found a hindi word I didn't recognize, I would still be able to tell if it were a name or not. Perhaps caps exist for purely stylistic reasons.

I wonder what would happen if someone did a design study of showing all caps vs sentence case text to people who have never seen English. Wonder which one would they say looks better.

You're doing yourself a disservice. As we all know, caps lock is cruise control for cool. All you're doing is throwing out the resumes of A-listers.

I remember learning about this at university.

The TL;DR version for those who don't want to bother with the links:

Full justification creates "rivers" of white space (effectively spaces above and below that partially line up forming a white line snaking down part of the page).

The eye has a tendency to follow these down the page which leads to either skim reading (you just resume reading where the river stops) or having to go back and reread and not get caught by the river this time.

Either obviously compromise the information you're taking in.

How do you mean how this applies to languages like chinese and Japanese? Do you mean that Japanese and Chinese have capital letter, or are you referring to english letters written in all caps read by a native Japanese/Chinese speaker?

In Japan(I don't know about china) it is very common for english words to be written in all caps when surrounded by Japanese characters. they will also (in many cases) be written in full-spaced letters, which only have capital versions: ＡＢＣＤＥ vs ABCDE. I've never seen it cause a problem, but I think this is more because Japanese speakers are not native english speakers, so their hesitation with an unfamiliar script would outweigh any recognition delay from it being written in all caps.

However, I do know that sentences that are written purely in katakana are much slower for native speakers to read, as it's a script that is used much less than kanji or hiragana. I would hesitate to say that reading an all-katakana sentence is more troublesome for a native jp speaker than a native english speaker reading something written in all caps.

Since there is no concept of CAPS in Chinese, I assume you mean that Chinese and Japanese characters are more or less square-shaped (thus less "shape contrast" from the linked article), are they harder to read? The answer is probably no for natives, but yes if you are used to latin-based languages.

One thing to note though is that there are so-called half-width and full-width symbols in most Chinese fonts (I think it's the same in CJK, but not very sure).

These are half-width symbols (normal symbols):

1234567890 ALL CAPS sucks

There are full-width counterparts (if your system and display them):

１２３４５６７８９０ ＡＬＬ ＣＡＰＳ　ｓｕｃｋｓ

Half-width and full-width chars get their names due to the fact in a monospace Chinese font, two half-width chars will take the same space as one full-width char. This makes alignment easier.

By now it's pretty safe to say using full-width chars is a bad idea. The readability is so low and mixed Chinese and English content is so common that people don't tolerate the ugly full-width chars any more. The norm right now seems to mix monospace Chinese glyphs (all Chinese chars have same width) with proportional latin glyphs. Unfortunately most Chinese fonts come with really ugly latin glyphs, and the remedy is to specify a latin font first (Arial and Verdana are the two most popular choices) and fallback to some Chinese font.

was going to write same thing. Contrast isn't everything.