How to work with key signature training
The module shows a key signature at the start of a staff and asks for the key it belongs to. The first levels stay with few accidentals and major keys only; later all seven sharp and seven flat keys appear, and after that the relative minors.
The task is pure recall under time pressure, and that is exactly what makes it valuable: key signatures have to be grasped in a fraction of a second while sight-reading, not worked out. Anyone reliably under two seconds here has one worry less when playing.
Use the resolution after wrong answers actively. Look at which accidental comes last and run the rule through your head before clicking on. Without that follow-up the exercise is a test, not training.
The order of accidentals
Accidentals never appear in arbitrary order. Sharps follow the series F♯, C♯, G♯, D♯, A♯, E♯, B♯; flats follow exactly the reverse: B♭, E♭, A♭, D♭, G♭, C♭, F♭. Both series move in fifths, upward for sharps and downward for flats. That is precisely what the circle of fifths makes visible.
Two shortcuts follow. In sharp keys the tonic lies one semitone above the last sharp: with F♯, C♯, G♯ the key is A major. In flat keys the second-to-last flat is the tonic: with B♭, E♭, A♭ it is E♭ major. The one exception is F major with its single flat, which you simply remember.
Every major key shares its signature with a minor key, the relative minor, a minor third below. C major and A minor have no accidentals, G major and E minor one, F major and D minor one. So the signature alone never tells you whether a piece is major or minor; the final note and the harmony decide that.
From practice
Draw the circle of fifths once by hand, with every accidental in place. Drawing it takes ten minutes and replaces hours of memorising, because the structure becomes visible: every step clockwise adds a sharp, every step anticlockwise a flat.
For everyday use, automate the frequent keys first. G, D, F, B♭ and E♭ major cover most of the literature for guitar, piano and choir. F♯ major with six sharps is allowed to take longer, because it appears less often.
A practical side effect: anyone who reads key signatures fluently transposes more easily. The question "how does this sound a fifth higher?" becomes "which key is one position further round the circle?", and that one has an answer.
Common mistakes
The most frequent mistake is confusing a major key with its relative minor, since both share a signature. On higher levels the module asks about this deliberately; read the question carefully.
The second mistake is counting accidentals without looking at their position. Three sharps mean A major only if they really are F♯, C♯ and G♯, in that order and on the right lines. A misplaced signature is rare in real scores, but looking closely is part of reading.
The third mistake is mixing up key signatures and accidentals within a bar. What stands after the clef at the start of a line applies to the whole piece; what stands before a note mid-bar applies only to that bar. It sounds trivial and is one of the most common sources of error for beginners.