I've never seen a ballet at the theatre, and only occasionally on TV. I never took a ballet lesson in my life. Nevertheless, I know a bit about it from a childhood spent reading, as well as other things, ballet and theatre books by authors such as Noel Streatfeild and Pamela Brown. I absorbed ballet terms and techniques along with directions on how to fire a gun and go fly-fishing (The Islanders), what you might add to bread (No Entry), how to mend a leaky boat (The Goodbye Island), ride a horse (multiple titles) and fly a falcon (Falconer's Lure). And so it went on.
For a while my cousins learned ballet, and I remember being shown how to stand in first, second and third positions. In fact, I can still do it!
This is the kind of situation that means I can write in books such as Just Like Emily about someone whose friend takes ballet lessons. I wouldn't be able to write a long book that centred on ballet, but I know enough... There's a lot to be said for cousins and childhood reading.