Why double row of pin headers?

I bought a simple SD card breakout board for a project, and it has a double row of pin headers, with each I/O having two pins (the I/O are GND, +5V, +3V, MISO, MOSI, SCK and CS). I hadn't seen this before and it got me thinking about why you would do this. The only reason I could come up with is if you wanted to chain multiple boards together, but that wouldn't make alot of sense in this case.

Think a little more SD save’s data you’ll want to add something to make data GPS SIM or something.

Some SPI lines connect to multiple devices. Those devices might want power as well. This makes it easy to connect devices in a chain.

rwiens: I bought a simple SD card breakout board for a project, and it has a double row of pin headers, with each I/O having two pins (the I/O are GND, +5V, +3V, MISO, MOSI, SCK and CS). I hadn't seen this before and it got me thinking about why you would do this. The only reason I could come up with is if you wanted to chain multiple boards together, but that wouldn't make alot of sense in this case.

Could be just for more mechanical strength when soldered to a PCB. As such a module is subjected to some mechanical forces as users install and remove the SD chip, there would be much better resistance to bending forces with a duplicate set of header pins soldered to the board.

Lefty

It will be for an insulation-displacement connector on ribbon cable:

http://en.wikipedia.org/wiki/Insulation-displacement_connector

[ on second thoughts that's not so clear here, but it is often a reason for dual-row connectors ]