XD Picture Cards
The XD Picture card is a Flash Memory card developed by Olympus and Fujifilm and is mainly used in Digital Cameras. There is limited use of the XD Picture Cards in other devices. XD stands for eXtreme Digital and was a very fast card at the time of development.
There are 3 different types of XD cards - the Standard, M and H. The Standard card has a limited capacity to 512Meg, while the M and H cards have higher capacity. The M card was developed to be a higher capacity card. The H card is a higher speed version of the M card.
To determine the type of card if there is an M or a H on the bottom left side of it - you immediately know what sort of card you have. The standard card has no letter designation.

