Build box shaped territories
Rectangles are more efficient than crooked shapes to build territory.
Therefore, one builds square shaped moyos rather than rectangles rather than oblique shapes. Here we see how Black develops more efficiently than White. The marked Black extension is therefore the primary extension from his shimari. The White extension is the secondary extension from her shimari.
What's the most efficient rectangle ?
Alex Weldon: I think it's worth noting that, when building on the side, you get one side (the edge of the goban) for free. So then the most efficient shape is a rectangle of width twice its height, not a square (in the corner, you have two sides, so a square is better).
White has actually invested one stone more than Black (19 vs. 18), but Black has 32 points of territory to White's 30. Because the side of the board forms one side of the territory, stones played to increase width count more than stones played to increase height (unless the width is already much greater than the height). Perhaps that's why it's often better to extend along the third or fourth line in the opening and early middle game, rather than building one's moyos upwards. Of course, the fact that territory built higher up is easier to reduce and harder to defend is also important.
For box-shaped frameworks, see also