Metadata, Resources, and the Resource Description Framework 9 Since the XML description is just another way of expressing a graph, it is possi- ble to do exactly the same operations on the XML description as on the graph drawn on paper. It is somewhat complicated to go back to the graph from the XML representation, however, because the same XML representation can be drawn as several different graphs (all graphs which have the same format are the same to a mathematician). Because you are working with triples of information that contain relationships to each other, you can also filter out information with a much higher degree of precision than alternate technologies. If you have an attribute-value-table, you can filter out information based on the value of the attributes. If another site does  not  share  the  same  attributes  and  values,  you  cannot  use  information from that site in your filtering process. However, if both your site and the other site express the information about yourself in a format that relates the state- ments to you, you will be able to filter out information to create a much more precise selection of information for the user. This can, for instance, be applied to a site which creates a home page from two different sites (what might be termed a personal portal) based on the user s preferences. It can be used to describe the copyright restrictions you have set on the use of the information, and it can be used to describe the relationship of your site with the site provid- ing the personal portal. As you can understand, whether you call the arcs Evidence A or a horsetail does not matter to the underlying graph. However, the properties of the ele- ments can change the graph. You need to declare what elements you are going to  use,  and  which  properties  they  are  going  to  have.  These  properties  will change  the  way  you  can  make  statements.  They  will  change  how  the  terms interrelate, both in the current graph and with elements in other graphs. You need to declare the elements and their properties, and the structure in which they can occur, the schema of the graph. Schema in the present context refers to a data structure. It has gained that use in the database industry, where it describes the structure of a database table: the columns and their headings, and what restrictions there may be on the data in the columns. Both the properties and the RDF elements are objects in their own right, and can be the subjects of statements in themselves. This means that there can be graphs  chained  to  the  graph  you  look  at  orthogonally;  or  in  any  number  of dimensions, because to a mathematician, the paper drawing is not necessarily the clearest description. RDF Vocabularies A schema does not define only the data structure for your graph. It also defines the terms you can use in it, the vocabulary of the description. For instance, the 69528_CH01Ix  4/6/2001 8:15 AM  Page 9