By Samuel Thomas Houghton, Trainee Patent Attorney
Edited By Mark Houghton, European Patent Attorney, Chartered (UK) Patent Attorney
One of the key objectives of the patent attorney in drafting a patent application is effective claim construction. One of the key factors in this is the form of claim, and in particular the terms of the claim, those terms may be cast in structural language, defining specific features or functional language defining the way those features operate. This article provides a high-level overview of these concepts.
The dichotomy of structural claims and functional claims can be applied to most description. Where in normal language we may quickly and near arbitrarily pick one for any given description, when constructing documents to withstand scrutiny, especially legal document like patents, it is important to choose meticulously.
Structural claims are statements comprising of a set of compositional elements sometimes including their arrangement.
Functional claims are used for explaining what a role something plays or what it does.
Once again avoiding distracting legalese layout a second example: Structural: It is red. Functional: It is read. Nominal: A red book. 
Sometimes a mixed approach is used, with different sentences in a claim being of alternating type.
In common speech, we often have a default paradigm relating to what we are describing and our relation to it. The fabricator might not care what a component will do and the utiliser might not care what that the item he uses is comprised of; therefore the specifics of what they present and equivalent and interchangeable differ.
When in a legal document a sentence would naturally have a noun (a part of speech commonly with structural and/or functional connotations), where the author craftly wishes to be near exclusively structural or functional, then a placeholder noun like ‘means’ is employed. Then adjectives for structural claims and verbs functional claims constrain the nature of the claim. In the a very literal manner to say ‘the invention comprises means’ is a structural claim, but in practice it is mere syntax.
Now we are on the same page, you may ask the question: which is preferable?
The advantages and disadvantages of each, through the lens of patent drafting.
Advantages of structural claim language. Structural claim language is easy to police and interpret, even when not in use or the use is hard to recognise it can be clear quantify the presence, or lack thereof, of the constituent structure.
Disadvantages of structural claim language. The primary risk when writing structurally is accidentally being too narrow, to say that something is made of metal is to say that no polymer, ceramic or composite can be used.
Advantages of functional claim language. Functional language can cover an unknown underling concept or hard to describe geometry. Whether obscured by being proprietary or being indescribable, functional language can be the only option. A large range of embodiments which are superficially different but functionally interchangeable can be covered.
Disadvantages of functional claim language. The primary risk of writing functionally is accidentally being too broad, to say a vehicle can be driven, or has means for propulsion not only includes cars and lorries but also a barrel boat  and paddle, such that any object that floats, even if never used near water, or any object that ever could push away water as a paddle is included.
In conclusion it is important to be aware of and effectively use each, likewise one can make inference about the thought behind the choice of language of even comparatively innocuous choices.
 "Structural," Merriam-Webster, [Online]. Available: https://www.merriam-webster.com/dictionary/structural. [Accessed: 2023-03-11].
 "Functional," Merriam-Webster, [Online]. Available: https://www.merriam-webster.com/dictionary/functional. [Accessed: 2023-03-11].
 "Nominal," Merriam-Webster, [Online]. Available: https://www.merriam-webster.com/dictionary/nominal. [Accessed: 2023-03-11].
 G. McKee, "Barrel Boat," Instructables, 2012. [Online]. Available: https://www.instructables.com/Barrel-Boat/. [Accessed: 2023].
 O. Gómez, "Eyeglasses on opened book beside cup of coffee on table," Pexels, 2019. [Online]. Available: https://www.pexels.com/photo/eyeglasses-on-opened-book-beside-cup-of-coffee-on-table-2846814/. [Accessed: 2023-02-11].