Secure Digital (SD), a memory card standard. Has multiple form factors (Standard, MiniSD and microSD/TransFlash) and multiple capacity ratings (SDSC, SDHC, and SDXC). Also extends to SDIO, which allows peripherals in an SD form factor to attach to a host, similar to USB.

Secure Digital (), a memory card standard. Has multiple form factors (standard, miniSD and /TransFlash) and multiple capacity ratings (SDSC, SDHC, and SDXC). Also extends to SDIO, which allows peripherals in an SD form factor to attach to a host, similar to .

microSD is a physically smaller version of the Secure Digital (SD) memory card standard. It's currently the smallest form factor in the range with dimensions of 15 × 11 x 1 mm. Electronically, it is identical to full-sized SD cards and MiniSD cards.

MMC cards are similar, and pin compatible, though the protocol is different. SD competed with CompactFlash cards, though it has widely won out as the go to flash card standard.

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