Jill bet, Tommy called, and Julie overcalled.
Hence bidding no-trumps in this auction shows a very strong hand, too good to overcall 1NT on the first round.
Some players also use the responsive double when partner has made a simple overcall in a suit.
An example would be a player with 11 points whose partner opens in third seat and hears a 1NT overcall on his right.
Our N/S pair settled for 6, after a diamond weak jump overcall from East.