[X-Istence]

AMD was the first to market with AMD64 ... no surprise that they got to slap their brand on it.

[jeltz]

Just like Intel named their competing 64 bit architecture Intel Itanium. The AMD64 architecture won and Intel decided to license it.

[potkor]

In practice Itanium only competed with the existing 64-bit architectures from the Unix vendors. Many of whom got suckered into dropping their own stuff and moving onto Itanium, and got wiped out because of the very late & underperforming Itanium chips (frequently called "Itanic")...