In the world of portable memory, Sony Ericsson only provides you with one option when it comes to most of their cell phones: Memory Stick Pro Duo.
Memory Stick Pro Duo
This format fits right in the middle of Sony's Memory Stick line, being physically smaller than the standard Memory Stick (most often used in Clie PDAs and CyberShot digital cameras), while being quite a bit larger than Memory Stick Micro cards (also known as M2). In this way, they can be thought of as being equivalent (both in size and capacity) to miniSD cards, whereas the M2 is on par with the fingernail-sized microSD.
Memory Stick Duo vs. Pro Duo
Both Memory Stick Duo and Memory Stick Pro Duo formats are physically the same size, and they are typically interchangeable in the devices that use them (like Walkman phones). That said, there are distinct differences.
Just as Memory Stick Pro cards are faster (both reading and writing), more capacious, and -- as a result -- more expensive than garden variety Memory Sticks, their Duo variants compare to one another in much the same way. Oftentimes, you can find "high speed" versions of Memory Stick Pro Duo cards, and these work much better with devices that require higher access and transfer rates. Most notable in our case here are high-megapixel Camera Phones and powerful music phones.
A Propriety Format
Sony devices, and Sony devices alone make use of Memory Sticks and Memory Stick Pro Duo cards. This is very unlike Secure Digital and Compact Flash memory. As such, when you pick up a Sony Ericsson cell phone with expandable memory, there's a very good chance that it implements Memory Stick Pro Duo cards (or perhaps Memory Stick Micro).
As such, the first company that comes to mind when considering a memory card for your Sony (or Sony Ericsson) electronic handheld is Sony itself.
Fret not, discerning consumer! You have options! Sony does not have a monopoly on the format. Two very prominent memory card makers also produce Memory Sticks (and Pro Duos) and these are Lexar and Sandisk.
Typically, when you purchase a Memory Stick Duo or Pro Duo card, it will come with an adapter that essentially transforms the storage card into a standard sized Memory Stick. This way, it will fit more easily in those multi-card readers that accompany computers (both laptops and desktops) these days. Without this adapter, you'd have a difficult time transfering pictures or music from your powerful mobile phone onto your PC.
Mobile Phones that Use Memory Stick Pro Duo
The only cell phones that you will find that make use of Memory Stick Pro Duo cards to expand their memory are those created by Sony Ericsson. Their most common application is in the company's Walkman line. Popular handsets that use the format include the W900, K750, W700, and P990.
Being Phased Out?
Newer Sony Ericsson cell phones are using the Memory Stick Micro (M2) format, just as several manufacturers have jumped from using miniSD cards in favor of the smaller microSD format. New phones that use M2 cards include the Z558, V630, W710, and Z550.