pixel coordinate ref. point?

[Bruce Wylie]

I was wondering what reference point within the pixel Grass applies
the coordinate system to. I want to transfer a Grass image to ArcInfo
and need to know if the coordinates given in the cellhd file are for 
1) the center of the pixel, the lower left corner, etc.

I realize that this info is probably in the programers manual but I 
currently do not have access to one. 

Thanks,

B. Wylie