About this tool
Choose a quality (major or minor) and a tonic, and this finder shows the key signature straight away — how many sharps or flats it has, exactly which notes are altered, the relative key, and its two neighbours on the circle of fifths. Everything is looked up from a built-in music-theory table in your browser, so nothing you select is ever uploaded.
Sharps are always added in the fixed order F♯ C♯ G♯ D♯ A♯ E♯ B♯, and flats in the mirror order B♭ E♭ A♭ D♭ G♭ C♭ F♭. Each step clockwise around the circle raises the tonic by a perfect fifth and adds one sharp; each step anticlockwise lowers it by a fifth and adds one flat. The relative minor sits a minor third (three semitones) below the major and shares the same signature, while the dominant (one fifth up) and subdominant (one fifth down) are the immediate neighbours that share all but one accidental.
Use it to transpose, to build chord progressions, or to answer "how many sharps does E major have?" at a glance. The dominant and subdominant are the keys you most often modulate to, and the altered-notes list tells you precisely which piano keys to play.