The 3 and 6 credits system in University courses?

I wonder if I posted this question before but because I did not see it on Answers, I decided to post it again. Now, about a week ago, a friend of mine at work asked me why the University courses were given in numerals of 3s and 6s. Now that question has also got me wondering about it. Why are course credits given numerals of 3s and 6s instead of 1s, 2s, 4,s or even some other numerical forms. Is there any signifincance to this systerm being carried out in many Universities? I'd appreciate your answers.