I'm adding to existing solild hardwood 2-1/4" strip oak flooring, where the new boards must be perpendicular to the old flooring. After sawing the ends straight, shouldn't I use a router to create a tongue on the end of the old flooring strips before laying in the starter board? I don't see how my combination tongue and groove router bit can properly cut the old boards while still in place. I would like to set myself up to make tongue and grooves, rather than simply face nail in such situations.

This is the router bit you need. It will cut a groove in a floor that has already been installed.

Top Mounted Bearing Slot Cutter Router Bit

http://www.mlcswoodworking.com/shopsite ... _slot.html

Thanks for the photo. Actually, I just came across the "Easy Groove", but I can see only in part how it works for form either tongue or groove. Can you supply a description of how it works. I can see though that there isn't a bolt a the bottom to get in the way, like the Sears one I have does.

It's used to make a groove only. Once you have a nice straight edge, you adjust the height of the bit to cut a groove in the flooring that matches up with the tongues. It is not for making tongues. To do that, you buy what is called spline, or slip tongue, and glue it into a groove.