I've often heard The Hitchhiker's Guide to the Galaxy referred to as H2G2 but never really understood why. I wondered whether anyone actually knew or whether it was just adopted unquestioningly.
- "The Hitchhiker's Guide to the Galaxy" is a pretty long mouthful of a name. Imagine having to say all that every time you wanted to talk to fellow fans about the franchise.
- The most obvious way to shorten it is to use initials (as is done e.g. with LotR for The Lord of the Rings and many other things), but in this case even HHGttG is a bit of a mouthful.
- Removing the minor words like "to" and "the" (which is also common practice) leaves us with HHGG, as seen on e.g. the relevant URL at Douglas Adams's website.
- And HHGG is quite a pretty little acronym, with two H's and two G's. It seems a natural step to go from there to H2G2, counting the letters rather than actually saying H and G twice.
You already know what H2G2 stands for, and that it is a type of acronym to avoid having to write or say the whole name. I think you are asking why the acronym is H2G2 instead of HHGG, which is the same length so there is no additional economy being made here.
The numbers 2 indicate repetition. The practice probably comes either from mathematics (algebra), where the numbers are written as superscripts (eg X^2 meaning X times X) or as prefixes (eg 2X meaning X plus X); or from chemistry, where they are written as subscripts.
For example H2O (or HOH) is the chemical symbol for di-hydrogen oxide (water) : 2 hydrogen atoms attached to 1 oxygen atom. Or H2SO4 (or HHSOOOO) which is the chemical symbol for dihydrogen sulphate (sulphuric acid).
So H2G2 is the 'chemical symbol' for (The) Hitch Hiker's Guide (to the) Galaxy. By the same rule, W3 or WWW means the World Wide Web; and TW3 - more correctly (TW)3 - is TWTWTW which means That Was The Week That Was (a satirical TV program of the 1960s). Following the mathematical usage, 3i means Investors in Industry plc, and 3M means the Minnesota Mining and Manufacturing Company.
There are several reasons for such numeronyms. Beside repetition, numbers are also used phonetically (eg K9 for "canine"), or visually (eg H4CK3D for HACKED), or to indicate the number of letters omitted from a long word (eg i18n for "internationalization").
In the present case the intention is probably to give the acronym a more scientific or modern appearance, similar to Y2K for "the year 2000", making use of the decimal abbreviation K for kilo = 1000, instead of the (ancient) Roman numerals MM.
It was the title given to the "Earth Edition" on the web:
This was founded - and named - in 1999 by Douglas Adams himself.
Clearly taken from The HitchHiker's Guide to the Galaxy. This website was an early attempt at crowdsourced knowledge. Somewhat overshadowed by the later Wikipaedia.
From 1978 up to about 1982 I had a friend who worked in the sound department of the BBC. The series was commonly either referred to Hitchhiker's or HHG. The abbreviation H2G2 become more common after the creation of the website.