Each end is cut into a dovetail shape, with 1" long shoulders behind them. These fit right into corresponding spots on each tie beam.
To the left, you can see some of the completed pieces. Unfortunately, you can also see some of the blue stain. I had these things stacked and stickered, but mother nature decided it needed to rain virtually every day. What can you do, I'm not making new ones. I will say though that when the beams get oiled and some sun on them, the staining doesn't look too bad. Most of the time anyway.
Here are some of the joists installed. They went together really easily (especially with a crane). They are 31" on center. Per one of the books I have, 30" is what you should go for from an aesthetic standpoint anyway. 31" is close enough, and gave me even spacing.
This is the top view of an installed floor joist. When final assembly comes along, you drive a wedge on each side of the dovetail. This will pull each piece tight with the tie beam, and ensure that the joists never move.