No Mercy is a PPV on the Smackdown brand. It is Smackdown's first PPV after Summerslam and usually takes place right after Unforgiven. It only has four matches (In Smackdown Vs Raw 2008) due to having an Elimination Chamber match as the main event.
The show starts with three matches on it's pre-show, Velocity (in Smackdown Vs Raw 2007). The show has only four matches (in Smackdown Vs Raw 2008). It has an Elimination Chamber match in the main event, similar to Unforgiven.