Simple Key

The Simple Key is a key that opens simple locks, but due to its shoddy construction and the damage caused by age, it can only be used once.

Uses
There are three locations in the game that can be opened using Simple Keys. By descending order of importance, those are:
 * The entrance to the Royal Waterways, which is required for game progression.
 * The cave to the right of Dirtmouth, beyond the graveyard. This cave holds Confessor Jiji, who holds the ability to summon your Shade to you in exchange for a Rotten Egg, which is useful if your Shade is stuck in a hard-to-access area and holding a lot of Geo.
 * The Pleasure House in the City of Tears, which holds hot springs and Songstress Marissa. It's also Millibelle's refuge after she scams the Hollow Knight. This location is far less important than the other two and should typically be unlocked last.

Locations

 * One can be bought from Sly in Dirtmouth for 950 Geo.
 * One can be found in the City of Tears, in the big room with multiple flying enemies and a Dream Tree just below the City Storerooms. The Key is located in one of the alcoves on the right of the room.
 * One can be found in the Ancient Basin, on the body of a Royal Retainer inside a giant dead Mawlek.