Grade in bolts

10.9

10 = 1/100*Ultimate strength In this case US=1000 N/mm² (10=US/100 = 1000/100=10)

9 means that yield strength is min 90% of ultimate in this case 0.9*1000=900 N/mm²

In fact values are higher as indicated in the standards since specified are minimal values.

The most used classes are 8.8/10.9 and 12.9. There are also lower classes but used only in screew for wood never in mechanical constructions.

Although not used in mechanical engineering, lower grades are still used in structural engineering e.g. 5.8 and 6.8.

I haven't seen 4.6 used for a long time (only back when I came out of college) but that doesn't mean to say that they don't exist still in some parts of the world.

Structures have a very high degree of redundancy in the bolted assemblies this is the reason to use also low grades, but the trend with bots as TC and others is to go to higher grades for different reasons one of them being the fact that a higher grade leads to a higher ratio L/d and thus offers for same preload a higher degree of reliability with respect to preload/time. Most structural assemblies bond thin parts so that a slimmer bolt is welcome.

This may interest.