ECMA Office Open XML ("Open XML") is an international, open standard for word-processing documents, presentations, and spreadsheets that can be freely implemented by multiple applications on multiple platforms.
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The Open XML format container is based on the simple and parts-based compressed ZIP file format specification. At the core of the new Office XML Formats is the use of XML reference schemas and a ZIP container. Each file is comprised of a collection of any number of parts; this collection defines the document.
Document parts are stored in the container file or package using the industry-standard ZIP format. Most parts are XML files that describe application data, metadata, and even customer data, stored inside the container file. Other non-XML parts may also be included within the container package, including such parts as binary files representing images or OLE objects embedded in the document. In addition, there are relationship parts that specify the relationships between parts; this design provides the structure for an Office file. While the parts make up the content of the file, the relationships describe how the pieces of content work together.
The results are XML file formats for Office documents that are tightly integrated but modular and highly flexible.
SDK articles and blog posts
Compatibility and converters
Open Packaging Conventions (OPC)