What are three main clues in The Hound of the Baskervilles?

1. The footprint. Sherlock Holmes discovers a single, gigantic footprint near the body of Sir Charles Baskerville. This footprint is clearly not human, and it suggests that the killer may be a large, savage creature.

2. The moor. The Baskerville family's estate is located in the middle of a desolate moor. This moor is known for its eerie atmosphere and its many legends, including the legend of the hound of the Baskervilles. Holmes believes that the killer may be using the moor to hide from view.

3. The letter. Before he dies, Sir Charles Baskerville receives a letter warning him that he is in danger. This letter is signed with the initials "L.L.," and it is written in a strange, archaic handwriting. Holmes believes that this letter may be a clue to the killer's identity.