Discovered Attack and Discovered Check
In intro to tactics I lightly introduced the concept of discovered attacks and discovered check. There I noted they were a type of double attack.
Here I am covering them in more detail because if you or your opponent misses one it can often decide the game instantly. As a reminder, a discovered attack is the action of moving a piece to attack another piece while simultaneously opening up a rank, file or diagonal to a piece of yours that is now attacking a second piece.
For example here black moves his bishop with check, while opening up the c file for his queen which is now attacking the white queen. As the queen is now in danger, white loses his queen after he moves his king out of check.