# QT atom container description key Build QT atom container-based data structures. ## Overview Because QT atom container–based data structures are widely used in QuickTime, a description key is presented here. Its usage is illustrated in the following sections, [Sprite media handler track properties QT atom container format](/documentation/QuickTime-File-Format/Sprite_media_handler_track_properties_QT_atom_container_format) and [Sprite media handler sample QT atom container formats](/documentation/QuickTime-File-Format/Sprite_media_handler_sample_QT_atom_container_formats). ```c [(QTAtomFormatName)] = atomType_1, id, index data atomType_n, id, index data ``` The atoms may be required or optional: ```c // optional atom // required atom atomType ``` The atom ID may be a number if it is required to be a constant, or it may be a list of valid atom IDs, indicating that multiple atoms of this type are allowed. ```c 3 // one atom with id of 3 (1..3) // three atoms with id's of 1, 2, and 3 (1, 5, 7) // three atoms with id's of 1, 5, and 7 (anyUniqueIDs) // multiple atoms each with a unique id ``` The atom index may be a `1` if only one atom of this type is allowed, or it may be a range from `1` to some constant or variable. ```c 1 // one atom of this type is allowed, index is always 1 (1..3) // three atoms with indexes 1, 2, and 3 (1..numAtoms) // numAtoms atoms with indexes of 1 to numAtoms ``` The data may be leaf data in which its data type is listed inside of brackets [], or it may be a nested tree of atoms. ```c [theDataType] // leaf data of type theDataType childAtoms // a nested tree of atoms ``` Nested `QTAtom` format definitions [(AtomFormatName)] may appear in a definition. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)