Clef
At the center of the image shown is middle C, at a frequency of 261.63 Hz.  Other names for it are C4 or c'.  It is located at the center line of the alto clef.  But on the bass or alto clefs it resides one ledger below or above the staff, respectively.  The two C notes that are above c' are shown to the left and right, on the bass and treble staffs.  The three clefs are staggered so that all three middle Cs are aligned on a single horizontal line.  To facilitate the teaching of these clefs, the lines of the bass and treble clef are marked. A mnemonic for remembering GBDFA on the bass clef is "good bikes don't fall apart".  For the treble clef, one can learn "every good bird does fly" for the lines marked EFBDF.