Yes, I know the difference between a square and a rectangle. But I’m not quite sure what a “square” of baker’s chocolate is. Seethis picture. Is a square the two rectangles together, or a single rectangle?
That’s one square. It’s in sections like that so you can easily break it into two half squares.
One square which is equivalent to 1 oz. I verified this many times when I was younger because I had the same question as you.
Baker here confirming this.
Thanks! 
The half-squares are a pretty recent innovation that has messed up a lot of handed-down recipes. A square had been a square since the Truman Administration, if not before.
Still, it’s better than one of my recipes, written out by an aunt in the 1960s, that calls for a “39-cent” container of Cool Whip.
Then there’s the question of what constitutes “one graham cracker” – the large rectangle? One of the two squares it breaks into? One of the two rectangles that the squares break into?