I believe the X before a note means you play a double sharp. This, in C Major key, if it is placed before b note,one half tone up takes the note to C. But you need to play an extra sharp (or half tone up), so you'd play C# because the Major scale pattern is: Tone,Tone, Tone, Semi Tone, Tone, Tone, Semi tone, tone (adding up to 8 notes of this scale). The semi tones for C Major scale will occur between E and F intervals and the B and C intervals of the scale. In a different key's scale, the Major key intervals would follow the same pattern but would, obviously, occur between different note names, because your tonic or starting note of the scale will be different.You need to be aware of how scales are constructed to know which note to hit when you see a double flat or double sharp sign on the notation. You can't simply play the next letter name up or down because, depending on the key you're in, this may result in some pretty unpleasant dissonant notes. Though some experimental musicians may like exploring the creative possibilities of such off sounding wrong notes.Ever struggled with learning a hard classical piece, hitting the wrong notes and then thinking: Hey I quite rather like that "new sound", even though I know it's wrong, not what the composer wanted played.