It is said that when (je)doch is used at the beginning of a clause in the sense of yet the subject-and-verb-inversion may occur optionally;
I am wondering which option is the norm? I.e. which is more frequent, inversion or non-inversion?
Before reading the other question, I was totally unaware that jedoch can be used as a true conjunction in zero-position of a main clause. Wiktionary does not mention this usage at all, Canoo.net mentions it but does not give examples. A DWDS search was successful resulting in a single definite example from the Grimm dictionary:
in seim gewalt
Claudius die jungkfrawen bhalt,
jedoch er sie verbürgen sol.
The way it is written makes it look very archaic. There may have been more in the different corpusses but skimming over them I didn’t notice any. Only finally on the Wortschatzlexikon of the university of Leipzig did I find examples of modern usage — or actually one single one:
Jedoch auch in Likören und Süßspeisen trägt sie zur (Gaumen-)Freude bei, sie belebt und entspannt. (Source: Reutlinger General-Anzeiger, 7th July 2010)
Frankly, I had to read that sentence twice and still think it is badly written.
Searching for the adverbial usage of jedoch — the one, where it can only occupy the first or later positions of a sentence — would give you a plethora of examples from all kinds of texts encompassing the entire range from internet chats (okay, that may be rare, jedoch is not exactly the lowest informal register) to high-class literature. Most of these examples (assuming the remaining sentence is well-phrased) do not raise a single eyebrow under German native speakers.
All things considered, the adverbial usage (first position) is overwhelmingly the norm nowadays and the conjunctional usage (zeroth position) is dying out.
Concerning doch, both possibilities exist and I would hesitate to name a predominant option too quickly.
I consequently used the terms zeroth and first position rather than inversion. The grammatical phenomenon of a subject appearing after the verb is not usually considered an inversion in the strict sense in German. This is because apart from the finite verb’s position which is fixed according to the type of phrase and potentially present nonfinite verb parts whose position is also fixed, the fragments that make up a sentence can be shifted around more or less at will.
The term inversion when applying it to German should be reserved only for questions (verb first) and the verb-first construction of conditional subordinate clauses (in a more formal register):
Gehst du heute in die Stadt?
Gehe ich in die Stadt, treffe ich meine Freunde.