The three Cs you’ll be asked for

• C4 — middle C· the C closest to the centre of the keyboard, usually under the manufacturer’s logo on the fallboard. Bottom ledger line of the treble clef, top ledger line of the bass clef — written music is anchored here.
• C3 · one octave below middle C. Typical left-hand thumb home for early pieces.
• C5 · one octave above middle C. Typical right-hand thumb home for early pieces.

From two-black-key-groups you already find any C from the 2-black landmark. Here you go further: the specific octave matters, because hand-positions and staff-notation are octave-specific.

Why octave-specific Cs

Reading lessons (treble-staff-landmarks, bass-staff-landmarks, grand-staff-orientation) all anchor on middle C. Hand-position lessons (five-finger-pattern-c, hands-together- parallel) need C3 / C4 / C5 as starting thumbs. Once you can land any of the three within a few seconds, the rest of Weeks 5+ stops being “wait, which C?”