What are the possible options for the --arch option in debootstrap?

I need to set the arch option in debootstrap. So I did some research and read the manual. After reading the manual I see that the section on the options simply says --arch=ARCH Implying that I should know the correct syntax for the architecture I need. I don't. I need 64 bit architecture. I know that "i386" can be used for 32bit architecture. What should I set the --arch option to if I want 64 bit architecture? Or more generally what would the range of options be? I could guess (but don't know and can't determine) that potentially the range or arch options depends on the OS being booted. In my case its a version of ubuntu that I know should work in 64bit. So the question becomes how would I determine the 64bit architecture option syntax? I could further guess (but again don't know and can't determine) that the option syntaxes are actually supplied by the booted OS and if I knew where to look I could figure it out. In which case, where would I look?