How the yarn RKM valaue relates to yarn strength?

RKM is the short expression for "Reisskilometer" - "Breaking-kilometer", which is an unit out of date. This can be expressed by the "breaking force of yarn per kilometers" at which yarn will break of its own weight". This is equivalent to breaking load in g/tex.

E.g.: a Cotton-yarn with 15 RKM means, it would break of its own weight, if you wind off 15 km of the yarn and hang it up in 15km.