In reply to The Penguin Grid:
@Ryan Maelhorn:
And what's with this point, what it is it based on?
Well, note that there is a diagonal from the upper left corner to the lower right corner of the page.
Then, a line going from the upper right corner crosses that main diagonal at a 90 degree angle to indicate that point.
That means that that point marks where you could draw a horizontal line to create a rectangle, made from the top of the page above the line, which has the same proportions as the page as a whole - because its diagonal makes the same angle, except rotated by 90 degrees, as the diagonal of the whole page.
This rectangle is then the basis on which the three horizontal lines you do see drawn are constructed.
First, the point where the rotated shrunk page diagonal crosses the main diagonal defines the bottom-most of those three lines.
Then, a diagonal is made for the left half of the rotated page image, and where that crosses the main diagonal defines the second line.
Finally, the bottom-most of the three lines is used to define a new thinner rectangle; where its diagonal crosses the main diagonal indicates where the top line is to be drawn.
Ah: this might help:
http://ministryoftype.co.uk/words/article/constructing_the_grid/