Unification-based formalisms have been part of the state-of-the-art within linguistics and natural language processing for the past fifteen years. A number of such formalisms have been developed, all providing partially different constructions for representing linguistic knowledge. This development has been a benefit for the linguist and language engineer who want to develop a general natural language grammar, but the variety of formalisms makes it hard to find the most suitable formalism for a particular problem.

The goal of this thesis is to investigate the possibility of developing a platform for experimenting with different constructions within unification-based formalisms. To this end a meta-formalism, FLUF (FLexible Unification Formalism), has been created that allows the user to define his own constructions. This possibility is a further development of user-defined constructions as used in other formalisms.

While developing FLUF, the two properties of flexibility and predictability have been important as goals for the formalism. The property of flexibility allows the user to adjust the constructions within FLUF to the needs of his current problem while predictability concerns the computational behaviour and enables the user to adjust it to the current expressive power of the formalism.

The FLUF formalism consists mainly of three parts. The first part allows the user to define datatypes and functions on datatypes. This is similar to user-defined constructions in other formalisms, but here the user is allowed to affect the performance of the unification algorithm in several ways.The second part adds typing and inheritance to FLUF. Also here the main idea has been to provide variants of typing for the user to choose from. The last part, which allows for the definition of non-monotonic constructions, is a feature that is not provided in other formalisms.

The thesis concludes with a description of a pilot implementation of a tool based on FLUF and some possible applications where this tool can be used. This implementation suggests that it would be possible to build a future tool based on FLUF, provided predefined modules can be used to achieve better efficiency for the system.