In order to use some data set, a user must have some information at his or her disposal that is not strictly included in the data set itself. This information, called ancillary data (and sometimes called metadata 20), describes the shape and size of the data types that make up the data set, and provides information about many of the data set's attributes, as well. OPeNDAP uses two different structures, to supply this ancillary information about an OPeNDAP data set. The Dataset Descriptor Structure (DDS) describes the data set's structure and the relationships between its variables, and the Dataset Attribute Structure (DAS) provides information about the variables themselves. Both structures are described in the following sections.