Sudoku Players' Forum (clone)

I'm a newb. There, I said it.

I read on anoter site about solving methods, and I'm not getting it. They mention that:

The 4 in c1r1 means that a 4 must be in c2 of box 7.

Why is that 4 reserved to box 7? I'm not seeing that.

According to the rule of sudoku, there has to be a 4 in every box. The site is saying that the 4 in r1c1 restricts box 7's 4 to being in the cells that are also in column 2 (the ones marked "4?".

I understand their language, but not their justification. For instance, I see that the 4 in c1r1 eliminates all of column 1 for a 4 in boxes 4 and 7.

But why is column 3 of box 7 not a possible candidate for the 4? Where's the restriction there?

...oh wait, I think I just answered my own question. It's circumstantial, isn't it? Only because there's only one available column for the 4s in box 7, and one remaining in box 4 that forces the 4 into c3r5.

Got it, thanks.

Exactly. It's nothing more than the fact that, in this case, the cells in column 3 of box 7 are already full. It wouldn't be the case otherwise.