Open diagonal
A bishop's value is the length of its unobstructed diagonal, not the square it stands on.
White to play
Bg2
On f1 the bishop sees one square. On g2 it looks down the whole a8-h1 diagonal.
What to look for
Follow each bishop's two diagonals with your eye until they hit something. A bishop that stops after one square is doing nothing.
When it applies
Before you move a bishop anywhere. Ask which diagonal it will see from the new square, not whether the square looks safe — an unblocked bishop nine squares from the enemy king is a real attacker.
How it goes wrong
Your own pawns are the usual blockers. Putting pawns on the colour your bishop travels on turns it into the piece players call bad, and no amount of manoeuvring fixes that while the pawns stay.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.