Staggered Pin Gid Array. A 387 pin PGA more commonly reffered to as Socket 8 -- the Pentium Pro. It gets the 'staggered' part because the pin arrangment is different on either side of the chip. It's hard to explain, so let me use my ascii ninja skills to show you:
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Hopefully that makes some sense. It was designed this way, presumably, so prevent the chip from being put in backwards, as that would kill it; the PPro, when they first came out, were almost $1000 USD each. It was investment protection, I suppose.