With the pegs half-glued in place, it was easier to see where things didn't fit. I am not above whittling down pegs a little to make pieces line up better; I overbuild enough that I can get away with cutting a corner sometimes. One of the pegs had to be sawed off, drilled out, and replaced. After those adjustments, the pieces fit together very well, and I went on to the measuring and drilling needed to fit the rest of the side pieces together. It is starting to look like a chair now.
Some of the difficulties arose because I chose to make the chair the same way I made the couch: with the ability to be broken down into sides, seat, back, and cushions for moving, by pulling a few bolts and taking out a couple of screws. My next step is to drill the rest of these bolt holes.