Converting amount of spice seeds to ground

[bxgirl]

If a recipe calls for 1 teaspoon of cumin seeds or 1 teaspoons of mustard seeds, how much ground spice should I use? Is it a direct one-for-one substitution, or do I use more (or less) ground than seeds?
This has always puzzled me, and I know I'll get an answer here!!!
Thanks

[bxgirl]

Thank you all for your quick and sensible responses!!

[MikeG]

PS: It might work with the funny shaped stuff in large amounts, too, but not in "spoon" measurements.

[MikeG]

Diito - rather than possibly make an idiot of myself, I'll skip what I *think* explains it and just agree that for small, regular-shaped spices, the measures are all but invariably the same (unless you tap/tamp the ground down hard, which would give incorrect results with pre-ground spices too.) It doesn't hold true for things like cloves, whole cardamom pods and other very irregular spices, though.

[King of Northern Blvd]

FWIW, I just do a 1-1 substitution. I may not be %100 correct but I have never screwed up a dish by doing so.