Of course you have to use both a stacking AND a resolving gel. The stacking gel will "stack" the sample you loaded in a sharp band, which will the be separated in the resolving gel, according to their MW. Without the stacking gel, your sample would separate as a smear and each protein could appear as thick as the well height...
The use of a continuous gel (increasing in acrylamide concentration) is, for me, just useful if you want to resolve two (or more) proteins very close in mass.
Hope I could help..
Edited by Simonsays, 09 February 2005 - 08:11 AM.