No, I mean braces.
One of the ways that this stool can withstand the weight of a human (some more substantial than others) while only being made of 3/4″ white pine is to rely on braces under the treads. A third brace near the bottom in the back keeps the sides from splaying out under load. You could make these rectangular and rely on screws to hold them in place. However, making them with a dovetail shape really isn’t any slower, and makes the joinery more secure.
Once the braces are cut out, we can cut out the recesses in the side. IT IS IMPORTANT that you get everything in the right alignment, so be sure to mark everything clearly and uniformly so that you don’t have any mistakes. It will be extremely disheartening to realise that you accidentally cut things backwards, especially since it will invariably be the last one that you mis-cut. Go ahead, ask me how I know this…
We could, if we wanted, cut a dovetail on both sides of the joint. But cutting in only on the one half of the joint not only keeps us from having a little tiny half-pin up in the corner, but it is faster and easier, and even helps to keep everything square during assembly. It’s how the old masters did it, so it’s what I do.
Your joints will get screwed in later, but they should fit closely enough to hold together under friction alone. This can be a tricky joint to pull off cleanly, since there are so many show surfaces and interactions between the brace and sides. But take your time and pay attention, and you can persevere.