I argee with Patrolman Armes completely (read the related tip). I carried a back up gun in an ankle holster for years, until a more experianced, fellow officer reminded me of a very important point:
A back up is important to have should your primary weapon fail. However, a more likely occurence is an attempted disarming. We are trained to lock down on the sidearm with our gun hand in an effort to keep it in the holster. At the same time we are trying to fend off the attack and radio in for help.
At that point, a back up gun in a strong-hand draw ankle holster may as well be in the patrol car’s trunk.
At first, I slipped my PPK-S behind the trama plate in the front of my vest. It was too big to sit on all shift in my back pocket. When I changed back up guns to a heavier, thicker, Para-Ordinance P-10, I switched over to uniform shirts that had a zipper behind false buttons and a weak-hand draw body armor holster. I could access the back up gun almost as fast as I could get a primary out of a level 3 protection holster.