The concept of defaults is used frequently in linguistics, so it is important that it is well defined.
- Defaults are often considered as standard, 'unmarked' properties. For example in phonology, a default property of consonants is to be voiceless (Spencer 1996).
- Defaults can be overridden, i.e. they should hold for the core cases, but they are not strong constraints or principles. For example in phonology, of course, there are also voiced consonants, but this is considered a more 'marked' case.
Defaults in Head-Driven Phrase Structure Grammar (HPSG)
HPSG is a constraint-based grammar formalism. In its formulation in Pollard/Sag 1994 there were only strong constraints (or principles), and no defaults.
In more recent publications such as Ginzburg/Sag 2000 or Sag/Wasow/Bender 2003, defaults are used. One example of a default discussed on these wiki pages is the Valence Principle.
Defaults are usually used to simplify the formulation of rules. HPSG researchers that use defaults normally only allow for type constraints, i.e. all constraints live on a particular type. Researchers that do not assume defaults may use a description language that allows for more complex constraints, i.e. constraints that allow complex descriptions in the if-part of a constraint.