Secure Digital or (SD) is a non-volatile memory card format for use in portable devices. The Secure Digital standard is maintained by the SD Card Association (SDA). SD technologies have been implemented in more than 400 brands across dozens of product categories and more than 8,000 models.

The Secure Digital format includes four card families available in three different form factors. The four families are the original, Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. The three form factors are the original size, the "mini" size, and the "micro" size (see illustration). There are many combinations of form factors and device families.

Electrically passive adapters allow the use of a smaller card in a host device built to hold a larger card.

Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards, but older host devices do not recognize newer cards. The SDA uses several trademarked logos to enforce compliance with its specifications and assure users of compatibility. This article explains several factors that can prevent the use of a newer SD card:

"Choose the right" is a saying or motto among members of The Church of Jesus Christ of Latter-day Saints (LDS Church) that is taught to children and used by members of the church as a reminder to act righteously. The phrase is taken from an LDS hymn which has that title.

The letters CTR, a reference to this phrase, are incorporated into a shield logo designed by Helen Alldredge, a Primary General Board member in the 1960s. In 1970, a church committee headed by Naomi W. Randall recommended that the shield be incorporated into official church material. Since then, both the phrase and symbol have been used in religious educational materials for LDS youth of Primary age. The symbol is also used in LDS culture by members of all ages, both as a reminder of the motto, as well as an indicator of religious affiliation. The CTR initials, displayed on the shield and in other forms, can be seen mainly on CTR rings as well as other types of jewelry, tee shirts, bookmarks and stationery.

According to the US Patent and Trademark Office, the LDS Church's stylized "CTR" abbreviation and shield with the CTR symbol are trademarked for use on finger ring jewelry by Intellectual Reserve. The phrase "Choose the Right," however, is not trademarked in the United States and may be used by anyone for commercial purposes.^{[citation needed]}