When you're in the process of constructing a box, you have three kinds of lines:

  • Lines that have been drawn

  • Lines that have not yet been drawn

  • The line you're drawing right now

You can think of the first category as being pinned down on both sides. These cannot be changed. You can think of the second category as being pinned on one end, but free-spinning. Their orientation has not been established, but the edges still exist as part of the problem that you're solving. And finally, you've got the line you're drawing right now, which like the second category is pinned on one side, but whose other end you are in the process of pinning. In other words, you're in the process of establishing its orientation and setting it in stone.

In essence, what I'm saying in the section you quoted is not to only focus on what you see on the page right now. Thinking about all four edges simultaneously (with some that are pinned down and unchanging, but others that are still relevant to the problem, even if they're not fully pinned down) helps you think about whole sets of lines, and how they behave to converge together consistently. That is ultimately what this challenge is about - developing our ability to estimate the convergences of these sets of lines - and so we just don't want students to focus only on a smaller subset at a given time, as that is often a tempting prospect to speed things up.