How is [x] used in medieval German?

[Koen G]

The word for six, "sex" is a common one indeed, as are certain others like ax and ox. 

I think those are common in English, not in German. The point is that in German the respective common ending is "-hs" or "ck[e]s", but somehow it is changed into "-x".

I mean, as common as it gets in the sparsely populated world of native German words written with x :) Apart from the You are not allowed to view links. Register or Login to view. question, I do find this interesting in itself: why do the recipe books I checked use "x" only in some specific German words? It is absolutely clear that x was seen as a Latin phenomenon (numerals and words), so how did they decide which German words got to use it? In some cases, it may be because the word felt Latin, but certainly not all.

I went to the Swedish Riksarkivet website, which contains many transcriptions of charters, also in German. I filtered for German language and looked for usage of "x" in German words.

* dar bewise gi uns sunderlix guden willen ane -> This is the You are not allowed to view links. Register or Login to view. I found, certainly not a genitive, but it is a native German word.

* You are not allowed to view links. Register or Login to view.: frygdages vor pinxsten -> this is a Latin loan, but it's a tricky case since the Latin did not contain an x originally.

* The You are not allowed to view links. Register or Login to view. I found is more interesting:
"i schifpfunt spex" -> one schifpfunt (unit) of bacon (speck). If I understand correctly, this is a genitive form - specks - written with x. Seems like the goat is alive after all.

(Still these are very rare though - by comparison, Roman numerals are much more easily found).