Tongue & Groove Router Bits

Tongue & groove router bits seem like they should be pretty straightforward, but there are a multitude of variables to consider. What size material are you using?  Is it a squared joint or would you like a V in the groove?  Do you need it to be adjustable?  Will it be used for a rail & stile application, paneling or flooring? If it's for flooring, is it new or an existing replacement? If you have trouble deciding or have a special application, let us know and we can help.