All the key on the keyboard have something written on them which more or less describes their functionality, except for, traditionally, the Space bar.
Why don't the majority of keyboard space bar buttons have anything printed on them, potentially leaving users guessing what it does?
Like most the other keys, the space bar is marked with the glyph of the character it makes.
Tab and Backspace keys should be blank too. Unless you have a DAS keyboard (or severely worn down the keys) I doubt that's the case. This is not really an answer.
Commented
Feb 5, 2014 at 23:39
This wise question has its answer in Wikipedia (here: http://en.wikipedia.org/wiki/Space_bar), from which the above photography has been taken. According to it, the Space Bar Key has evolved from something that we would rather call a lever nowadays:
the "bar" was literally that, a metal bar running across the full width of the keyboard (or even wider, and even surrounding it) that triggered the carriage advance without also firing any of the typebars towards the platen. Later examples gradually shrank and developed into their current more ergonomic form as a wide, centrally located but otherwise apparently normal "key"
and the point was to make it similar to its ancestor, so people who got used to the old design would not be confused. Later on, it just made a tradition. Anyway, not every keyboard has an empty Space Bar Key, e.g. iOS touchscreen keyboard.
In addition to the legacy already mentioned, it's intuitive for the spacebar to not have any label because it produces an empty space. In other words, an 'invisible' label is more representative of an 'invisible' character.
The other keys on a keyboard either produce a visible character or perform some character/display modifying action. You could argue that the Tab button produces a set of empty spaces and ask why would it have label, but you could also argue that Tab is more representative of an action and not a single character.
Simply put, the "Space" label could've been confusing because it's both a noun and a verb.
Tab, Enter or Backspace (all "producing whitespace") are all "special whitespace", since the basic whitespace is simply a space. As well, what symbol would you use for that? My keyboard has a pair of arrows |<- and ->| on tab, an X in leftarrow on backspace, and a "stylish downleftarrow" on enter. What would you put on space?
Not all space bars are blank. I give you:
Which, okay, doesn't so much have a "bar" as a "key".
The origin of the keyboard goes as far back as 1873 when Christopher Latham Sholes invented the first practical typewriter and the QWERTY keyboard which we still use today. A beautiful piece of machinery where all letters had their equivalent on their keys - and the brown space bar had no printed text on the key.
Image from Cornell University and the Sholes Glidden (1874) article.
But there are limited resources explaining why the space bar is blank. We can only guess what Christopher Latham Sholes idea was, since he died in tuberculosis in 1890.
All the key on the keyboard have something written on them which more or less describes their functionality, except for the Space bar.
That is, simply, not true in the first place. I give you PCD Maltron, which labels its space keys as SPACE. Here's a close-up of a 2-handed ergonomic keyboard (L89 QWERTY layout):
Here's the Maltron mouth head stick keyboard, which (as with others mentioned here) has more of a space key than a space bar. It's still labelled SPACE:
(source: mkprosopsis.com)
Just for variety, here's a Kinesis ergonomic keyboard, again with the space key labelled Space. Kinesis doesn't use all-capitals labels:
It's not even true to say that old computer keyboards didn't label space keys, and that putting names on space keys is the sole purview of these new-fangled ergonomic chappies. The Commodore PET 2001 had a space key that not only was labelled SPACE but was labelled in red:
(source: oldcomputers.net)
The mistake of thinking that space keys don't sport labels is rooted in the misconception that there a thing universally known as "the keyboard".
If you look carefully, you'll notice that the space bar on normal modern keyboards has a distinct shape that distinguishes it from the other keys. Thus, no label is needed.
While Gregorz Janik's answer above explains why is is that way out of tradition, a possible additional reason is that the space bar a struck far more than any other key on the keyboard when typing text. As a result any text would be worn off quite quickly. Consider how quickly the spacebar gets "shiny" versus even the most frequently hit key (probably E).
