One-sided or two-sided PCBs?

I'm a little confused about the files for making PCBs. While the generic electronics build instructions talk about a "copper side", indicating that the print is one-sided, there are two "traces" files and two "silkscreen" files for each board. While I can understand silkscreening both sides, I don't get what the other "traces" file is for. The guides don't talk about the various files included, so I see no hints there. What am I missing?

-Lars

Some of the boards are actually two-sided. While that's not a problem in itself, the way those files are prepared poses a significant challenge for people who make their boards at home, like me. It just takes too much effort to line the two sides up perfectly using two separate sheets of paper, and any significant relative shift (or even worse, rotation) will ruin the board, usually in a way noticeable only after etching.

There is a simple way, though, to fix this. Just put the two images on a single sheet, side by side some 1-2cm apart, with a dotted line between the adjacent edges of the images, and with one image mirrored so that when you fold the print along the dotted line and put the board in the middle, the images should match properly, printed side in, ready for a few rounds in a laminator.

Now, the problem could be getting those images into a single PS/PDF file and positioned the right way. I'm using the ps2ps tool for that and there's really no other way I know of that doesn't involve rasterization. Certainly, Eagle cannot do that (it should, really) and I couldn't find any dedicated tool. Maybe I'll make one, I'll see into that in a few days, but I'm not making any promises. Should anyone do that first, they'd save a few people lots of grief.

Or just tape the two pieces of paper together lining them up using some lightsource (usually your window or for nerds monitor )

Also possible is first iron one side, then drill the mounting holes, use those to line the second side up and etch after ironing the second side.

'sid