Semi-final match ups.

Is it usually, in Grand Slams and other tournaments, in terms of which seeds are scheduled to meet:

1 versus 4, and 2 versus 3 OR
1 versus 3, and 2 versus 4?

Generally, which way is it, and which way do you think it should be? I think it should be 1 v 3, and 2 v 4.

It's random..

1 and 2 will always be in seperate halves and 3 and 4 will be in different halves.