The Simple Key in Hollow Knight is used to unlock doors with simple locks. These keys are consumable and break after use. Specifically, Simple Keys are used in several notable locations:

• Entrance of the Royal Waterways from the City of Tears
• Cave to the right of Dirtmouth, past the graveyard
• Entrance to the Pleasure Room
• Godseeker's Cocoon in the Junk Pit
• Confessor Jiji's home in Dirtmouth (to exchange Rancid Eggs for calling your Shade)
• In City of Tears (West) to open a hatch leading to the Royal Waterways
• In City of Tears (East) to access the Pleasure Room
• To free Godseeker from her bonds in the Junk Pit

These keys can be bought from the shopkeeper Sly in Dirtmouth or found in specific locations across Hallownest. In Hollow Knight: Silksong, Simple Keys are used similarly to unlock doors across Pharloom, with doors located northwest of Bone Bottom village, in Deep Docks, Sinner's Road (Green Prince's cage), and the Rosary room in High Halls. This use of Simple Keys allows access to secret areas, rare items, and new locations to explore.