Why fifty-minute classes?

[wnfrota]

Does anybody have any idea why, for the sake of counting credits (like one credit equals 15 fifty-minute contact hours), a single class lasts for fifty minutes, and not a full hour? I remember someone telling me it had to do with UNESCO conventions and/or recommendations... If so, I'd like to know which one/s...

[Tdol]

This site suggests that it might not be UNESCO: Credit