In a famous Sherlock Holmes case, the key clue was a dog that did not bark.

"That's one of the key clues in the entire case."

"Just why is that a key clue?"

I still cannot properly parse the key clue 15/13.

A key clue to the condition of wine is ullage: the fill level in the bottle resulting from evaporation.

Later, the nail gun's distinctive nails become key clues: every house boarded up with those nails contains a body.

The German epigraph at the beginning of the poem offers the key clue here.

If neutron and anti-neutron had differing half-lives, it would be a key clue to investigating that question.

But the key clue was East's double: it had been made only after a pause.