I würde immer "th" an non-integers anhängen

Frage: macht es Sinn Non-Integers wie Kardinalzahlen zu verwenden.
Hast Du den anderthalbten Apfel gegessen oder den zweiten Apfel erst halb???

First question is whether using decimal numbers as ordinals works at all ...

But if, you'd probably say "onepointfifth", hence "1.5th".

Just my 2 cents.

Would you not just say "position 1.5"? Or "Section 1, subsection 5" etc?

How would you say it in German?! 1.5.?

You would probably say "Der einskommafünfte", and I would write "1,5te" to make it clear, as "1,5." would look confusing.

When would you use this? How can anything be in half a position?
I must admit don't know how you can say this -- as Sophil has said, whether ordinals work

There is a difference between English and German in the use of the decimal places. In German you can say "zwei komma fünfundfünfzig", but in English you can't say "two point fifty five", you have to spell out the digits "two point five five".

Ghol, "zwei komma fünfundfünfzig" sagt man heutzutage eigentlich nicht mehr. Das kenne ich nur von älteren Leuten.

Emil: you cannot use -st, -nd, -rd, -th with decimals. The reason is, that ordinals refer to the order of discrete elements: the 1st among equals, the 2nd in a row, the 1000th time, the trillionth sand grain, the vigintillionth atom.

But, you cannot have the '1.5-th' sand grain, for which one would that be?

So the answer is, all your guesses are wrong, only natural numbers (1, 2, 3, ...) can take an ordinal suffix. (We'll leave to the computer scientists and mathematicians the question of whether the '0-th' element of an array, or the 'minus 3rd' element makes sense.)

Peter: Well said.
This naturally applies to all things u can't obviously split (countables) {u can split an atom, but it's not half an atom afterwards - but let's leave THIS to the physicians}.
However, my problem arises from this:
A product (fuel cell) has gone through several generations that were all distinct in one certain feature. Now, product development has gone back to improve the 2nd generation of cells making it a 2.5nd / 2.5th generation of cells since there is no distinctive difference to the 2nd generation and there are already successive higher ordinal designated to other newer models.
In German everything is much easier since there alswas is just the ...te Generation, the problem in English arises from the fact that the first three numbers are very different and thus, if one HAS to use (.te) in English there must be a rule somewhere to define whether that depends on the original ordinal or the last digit after the point.
Once again, there's a fix around that, namel saying generation 2.5, atom onepoointsevenbillion, ....


P.S.: For all those who have never heared of "ordinal":
http://de.wikipedia.org/wiki/Ordinalzahl

Now I see what the context is, I am sure the way that one would deal with naming generations is different. You'd probably do something like resort to Greek letters (alpha beta theta delta...) or Roman numerals.

Actually I secretly doubt whether that works linguistically in German... ;-)

like II.Vth? II.Vnd? II.V.?
Looks a bit strange, especially when everyone else uses arabic numerals (1234)
hm.

I don't know a great deal about fuel cells, but is "generation" the right word here? For other products I'm used to hearing "version 1.5" etc. A new generation of a product sounds like a completely different type to the last version of the old generation; you'd have the 5th generation of a product, but version 5.2.

I found 4 UK google hits for "2.5th generation" including:
"GPRS is one of the so-called 2.5th -generation services, the number 2.5 expresses the transition to the 3rd generation which will be represented by UMTS (Universal Mobile Telecommunications Systems). " http://www.tiresias.org/phoneability/bridging...
...but this turns out to be written by Germans, sorry. All the other hits are about the same phones by the look of it. The 2 hits for "3.5th" in UK also seem to be by non-natives.

Oh, the right word it surely is - or at least everyone uses it.
However, it is used (as you have sparsely observed) elsewhere as well.
It occurs to me that noone has ever really made a rule about this, except of course Peter's "there is no such thing" which might hold true - and is nonetheless not satisfying for me.