Loose piece
Undefended pieces are what tactics are made of. "Loose pieces drop off" — check every piece you own before moving.
White to play
Rxc6
Nothing defends c6, so there is no tactic to find — the rook takes the bishop and the game is effectively over.
What to look for
Point at each of your own pieces in turn and name what defends it. Anything with no answer is loose, and every tactic in the game runs through it.
When it applies
Before you commit to any move. This is the one check worth making every single time — most material lost below club level is lost to a piece nobody was watching, not to a clever combination.
How it goes wrong
A piece defended once is still loose if the defender is pinned, overloaded, or about to be captured. Count the defenders that can actually arrive.
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.