Got to talking about this play with an official that I work with. He agrees that the ball goes on the 12 1/2. He claims that is should be first and ten from here.
Here is the reason. When the USC penalty is enforced on the defense the ball goes to the 25. This spot gives the offense a first down. You then mark the 12 1/2 yards off to the 12 1/2 for the USC on the offesnse, from there it is 1st and 10 from the 12 1/2.
That's exactly right. When enforcing penalties on downs 1, 2, & 3, if any markoff puts the offense past the line-to-gain (as happened here with the first USC markoff), it will be first down. However, the chains are
NOT set until after all enforcements, meaning they will be set at 1st-&-10 from the 7.5 yard line. (Prior to 1997, the chains were set after the first USC enforcement, which would have given Red the ball at the 7.5 with a 1st-&-25.)
4th down enforcement can be different, depending on what fouls occurred and when. Assume that Red gains two yards on a 4th-&-three play.. Now, since the LTG wasn't reached during live-ball play, White will decline the clipping penalty, and it will be White's ball following enforcement of both USC fouls in order of occurrence.