We know that classical puzzle of getting 4 liters from 2 jugs of 3 liters and 5 liters capacity. (or was it gallons?) 

But what about other volumes? And what if the jugs had different volumes?

For instance, if you had a 7 liter jug and an 11 liter jug how can you get 9 liters exact?

Another one: If you had a 13 liter jug and a 5 liter jug, how can you get 9 liters exact?

Also, if these problems are solved before I come back again, what if you can make markings on the jugs? How does the game change?