First of all, shoutouts to the cowon X5. I've got mine around somewhere although I got it a bit after it's time.
Second of all, if you retroactively give it a number, you can put X1, X2, and X3 all down as Dance Dance Revolution games. This helps relieve some confusion.
And at that point you can make a number of reaches to make everything else music related. Except for X11, it's just been here since the dawn of time.
and the craziest thing is that Sigma is the villain here in every case
I thought these were describing the megaman X games and I was confused until I kept scrolling and they kept going up, and then I was REALLY confused until I saw x86
honourable mention: AX25, a protocol for packet radio over amateur radio bands.