Step on Line -- Legal or Illegal?

[Nevadaref]

Originally Posted by Juulie Downs

I know what the FT line is, Nevada. I thought the semi-circle was also illegal for the shooter to step on.

It's not. The FT must be attempted from WITHIN the semi-circle. Now consider exactly what constitutes the mathematical line of the semi-circle? It's the outermost edge, right? That's what it says on the court diagram in the front of the rules book. So the whole of the curved line marking this on the floor lies within the FT semi-circle.
The two following rules state from where the FT must be attempted and define the foot placement of the thrower. Admittedly, the second one could be written more clearly, but it does say BEYOND ... the free-throw semicircle line is illegal, not on it.
9-1-1 . . . The try shall be attempted from within the free-throw semicircle and behind the free-throw line.
9-1-3e. The free thrower shall not have either foot beyond the vertical plane of the edge of the free-throw line which is farther from the basket or the free-throw semicircle line.

Originally Posted by Juulie Downs

But that just emphasizes my point. The shooter can step on the line (but not over the line) going backward, or sideways, but not forward. How to explain THAT to a coach?!?

The best way to explain this to a coach would vary, but I would suggest that one start by asking the coach if the kid is positioned inside or outside of the 3pt area when his heels are on the semi-circle.
Then ask him if the kid is within the FT lane when standing with his toes on the FT line.