Can anybody tell me what are theses "X"

[Popon13]

http://puu.sh/AwrAI/b553e9a0a4.jpg

Hello I'm trying to learn "Gamma Knife" but I never saw theses "X" before, so I don't know what to do.

Does anybody know what I'm supposed to do, please ?

[firekorn]

https://imgur.com/a/vlxTS

 

Those are frethand mute chord.

[Popon13]

Ohhhhhh ok I see now, I feel a little stupid now because it's exactly the same cross as the other frethand muted cords, but I didn't make the connection.

Thank you very much for answering my question and giving me this little guide.