Our first schema is a very simple one. It contains three elements stuff, bit, and bob. As the names suggest, this is not a TEI schema, and it does not use either the TEI namespace or the TEI class system.

The schema is defined by a schemaSpec containing three explicit elementSpec elements, conforming to the definition provided for these elements in the TEI Guidelines. Note that the content models all make reference to the RELAXNG namespace, but are otherwise free standing. In particular, they make no use of TEI element or attribute classes.

Now suppose that we realise that our bit is much the same as the existing TEI p, our bob is much the same as the existing TEI ptr, and our stuff is much the same as the TEI div. Can we rewrite the above ODD to use just a few TEI elements?

Note the use of the source attribute to indicate the default source for any elements referenced by our schema specification. The elementRef element saves us the trouble of having to define our TEI elements; its effect is to copy into the schema the whole of the TEI standard element, including its namespace and class memberships.

Suppose, however, that having got accustomed to our stuff element, we wish to keep it, but to define its contents to use TEI elements. We can make a simple schema combining our new element within the TEI existing framework.

Here we did not explicitly state that the elements we want are to be taken from the current TEI P5 version, since that is the default behaviour. Note however that our non-TEI element must specify that it is in the null name space (or specify a name space of its own).

In the real world it is fairly unusual for schemas to have only a handful of elements. Partly for this reason, the TEI scheme is defined as a set of modules, each of which defines a number of elements. These modules are a convenient way of including several elements at once: Each moduleRef here could be replaced by an elementRef for all the elements stored in that particular module. The name of the module in which it is to be stored forms part of the declaration of an element in the TEI source. Element names are unique across all modules; module names are unique across a given source scheme.

It is sometimes convenient to say that one wishes to use all the elements from a given module, except for a small number. The attribute exclude may be used for this purpose:

Another way of achieving the same goal would be to add a new declaration for each element to be deleted from the schema, as follows.

This is a special case of the general mechanism for modifying elements discussed below. An ODD processor first gathers all the declarations for elements found within a schema specification, whether by explicit reference or by reference to a module. In the above example, it will therefore find two declarations for each of mentioned, quote, and said -- one from the module core, and one from the schema specification itself. In a second phase of processing, the ODD processor combines such multiple declarations according to specified rules, for example that if any declaration specifies mode="delete" the effect is to remove all declarations for that element.

An attribute include is also available on moduleRef for the convenience of those wishing to specify explicitly the elements required from a given module. The same effect could be achieved by means of an elementRef for each required element, of course: this redundancy is provided for convenience.

Note that when a schema is defined by inclusion rather than exclusion of elements it is important to ensure that any elements required by the content model of another element are actually present. In the current TEI Guidelines, content models are as far as possible defined in terms of element classes rather than explicit elements, which mitigates this problem. Nevertheless, there remain some obligatory elements, for example in the TEI Header, all of which must therefore be explicitly requested.

In summary, supposing element x to be provided by module Y, the following combinations are meaningful:

The elementSpec element is used to add a new non-TEI element to a schema. If that schema uses the TEI architecture as its base, then the schema will include all the declarations necessary to make use of the TEI class system. In the following example, we declare a new element diachrony in our own namespace and add it to the existing TEI dictionary schema:

For purposes of TEI conformance, it is required that any new non-TEI element explicitly indicate that it is not part of the TEI name space, using the ns attribute. To avoid possible name collisions during schema generation, it is strongly recommended that such elements also specify a unique prefix, using the prefix attribute.

In the following example, we add two non-TEI elements, admin and hi, both taken from the TBX name space. The admin element is added to the TEI model.hiLike class. Note that it is the non-TEI hi element which is referenced in its content model, and therefore a prefix must be used to distinguish the two.