@Paul
Actually, no. The first MemoryStick was limited to 128Mb, which they then kludged to 256Mb by making a switchable 2x128Mb design.
Sony decided then, in their infinite wisdom, to introduce MagicGate, their DRM enabled MemoryStick. You could have a MagicGate or non-MagicGate MemoryStick. Music players required cards with MagicGate to work.
Getting confused yet? At this point they introduced the smaller MemoryStickDuo cards. Which also came in two flavours, MagicGate or not.
To get around the 128Mb limit Sony introduced MemoryStick Pro, which unfortunately wasn't backwards compatible with older devices. That was also launched in Duo or not. At least they removed a layer of confusion here, as all subsequent MemoryStick Pro cards were MagicGate enabled by default.
So now we have six different and in many cases incompatible variants: MemoryStick, MemoryStick Duo, MemoryStick MagicGate, MemoryStick Duo MagicGate, MemoryStick Pro, and MemoryStick Pro Duo.
Some are physically incompatible, for example a MemoryStick Pro won't fit in a MemoryStick Duo socket as it's too big, others are electronically incompatible, for example a MemoryStickPro won't work in a MemoryStick 128Mb device.
Sony then decided to confuse things even further by launching some *more* electronic variants and physical types...
By this point the mainstream variants are MemoryStick Pro, and MemoryStick Pro Duo. Devices using the older types were rendered obsolete overnight without warning.
Now they've decided that Duo is too big, so they introduce Micro. We're now up to three physical standards. That's OK though, as they've introduced some new incompatibility types too. Mark2 and PRO-HG.
Then there are special MemoryStick variants, like the ones specifically for the AIBO.
MemoryStick is probably the single messiest memory card format out there.