[jcfields]

They're x, y pairs for the top-left and bottom-right corners of the rectangle. They're coordinates in the fourth quadrant of the Cartesian plane (i.e., with the origin conceptually in the top-left corner), which is also the way coordinates are given in most windowing systems (including X). This is more obvious when using the "poly" attribute rather than "rect," where you can have an arbitrary number of x, y pairs (and the browser automatically closes the shape by drawing a line between the last coordinate and the first).

The way CSS orders border, margin, and padding values is clockwise starting from the top. But these aren't coordinates since they represent sides, not points, so I think the author is confusing things by comparing them.

[LoSboccacc]

something I always wondered since the age of programming mode 13h, is the use of the fourth quadrant so that loop would fill video memory in the order the scan line goes reading it?

[toast0]

This is pretty much it. Scan lines are numbered from the top down, as they happen, everything else is numbered accordingly.

The real question is why scan top down and not bottom up? (I don't have any idea). Horizontal scan lines makes sense given a wider than tall screen, because it results in fewer scan lines, lowering the frequencies required ( a bit). Wider than tall makes sense for TV given common movie film aspect ratios and computer monitors started as TVs with different control circuits and cases.

[klodolph]

I think it’s just designed to match the English order for text. This way your text mode can go through memory linearly. Graphics mode can just reuse the same order.