Post History

Historically, books and magazines were printed on large pieces of paper, which were then cut into smaller pieces of paper.
Even today large reels of paper run through rotary printing presses  where the partial image printed in one color by roller needs to be lined up with the partial image printed in another color by another roller.

Some printing processes can't put ink all the way to the edge of the paper, and yet some artists want to put ink all the way to the edge -- they don't want a blank white border.
Again, this is often solved by printing on a large piece of paper, and then trimming down to the final desired size.

Often the artwork sent to the printer includes "printer's marks" or "crop marks" to tell the person cutting the page where to make the cut -- the "nominal" cut line.
However, the printer expects the actual physical edge of the paper -- after it's cut -- to be a few millimeters left or right from that "nominal" cut line.
Other kinds of printer's marks to help line up the various colors of inks on the page.
All of those printer's marks are eventually cut off.

Say an artist designs a poster that includes an image of green grass running all the way to the left and right edges.
On some prints of that poster, some of the details in the green grass are visible on the left edge, but other details near the right edge have been cut off.
On other copies of "the same" poster, those details near the left edge have been cut off, but other details on the right edge are visible on the poster.
This requires "extra" green ink ("bleed") outside the "nominal" cut line that is expected to be cut off and thrown away.

**MediaBox**: the size of the big piece of paper you expect to print on. The margins of the MediaBox (outside the Bleedbox) often contain printer's marks.

**BleedBox**: The area that includes all the areas that *might* end up on at least one copy of the final poster, taking into account the inevitable trimming misalignments. There's no point painting green grass outside this area. Typically a few millimeters larger in every direction than the Trimbox.

**Trimbox**: the "nominal" area that you intend to end up on the final smaller piece piece of paper, after cutting.

**CropBox**: not sure exactly what this is for; perhaps it's not that important?

**ArtBox**: The area that includes all the areas that *will* end up on *every* copy of the final poster, taking into account the inevitable trimming misalignments. Typically a few millimeters smaller in every direction than the Trimbox. (Also called the "safe area"). All text *should* be inside this area. Areas outside the ArtBox *might* be cut off on at least copy of the final physical poster.

When "printing" to a PDF file, some applications simply set all five boxes to exactly the same size.