Surprisingly, it is one of the most basic steps in creating this joint - sawing the edges of the tails. The top of the cut is relatively straight, but as the cut heads further into the wood it starts to bow and by the time I get to the bottom of the cut the tail has a distinct bow.
I'm using a 10" dovetail rip saw which is sharpened to perfection so I know the fault only lies with me. I can only assume that I'm not only moving the saw back and forth but side-to-side as well. I slowed down considerably on the next set I cut and tried to be conscious of my arm movement but it didn't help. The second set was just as bowed at the bottom of the cut as the first.
Does anybody have any suggestions as to how to overcome this problem with this very basic process?