Commas, Hyphens and “Which”

Used incorrectly, these three elements of writing can introduce ambiguities, and the potential for subsequent misunderstanding, into your writing.

Using commas and hyphens

In the sentence "Because Aβ42 levels were elevated in 75% of AD patients in studies using our method [6,7], it is critical to obtain fresh samples," moving the comma after method to follow the word “patients” (or addition of a new comma there) would completely change the meaning.

Similarly, in the phrase “calcium-induced calcium release,” omission of the hyphen completely changes the meaning of the sentence. When the hyphen is present, "calcium-induced" is a compound adjective modifying the noun "calcium release." When the hyphen is absent, "induced" is a verb describing the effect of calcium on calcium release. Thus, it is critically important to use hyphens with such compound adjectives to avoid misunderstandings. However, no hyphen is required to combine an adverb and an adjective. For example "highly intense staining" and "high-intensity staining" are both correct, but "highly-intense staining" is not.


  • "Glutamate receptors mediated synaptic plasticity..." tells the reader that Glu receptors are involved in the development of synaptic plasticity.
  • "Glutamate receptor-mediated synaptic plasticity..." identifies synaptic plasticity involving Glu receptors as the subject of the sentence (note the change from plural to singular because "receptor" is being used in a general sense and not to refer to a single receptor).

Using "which"

The word "which," when used incorrectly, can also induce considerable confusion. It is often confused with the word "that." Both introduce clauses that modify nouns, but "that" should be used to introduce defining or restrictive clauses and "which" should be used to introduce non-defining or non-restrictive clauses.

For example, in "the sections that were positive for GFP were subjected to cell counting procedures," the "that" introduces a defining clause that defines exactly which sections were subjected to cell counting. By contrast, in "the sections, which were positive for GFP, were subjected to cell counting procedures," the sections that were subjected to cell counting are rather loosely defined, possibly referring to sections that have been described in the previous or recent sentences. The clause about GFP positivity provides the reader with additional information, but is not essential to understand the meaning of the sentence.

Because "which" is used in this way, writers need to ensure that it is absolutely clear what the "which" is actually referring to. This could possibly be whatever immediately precedes it (most common), or possibly the main subject of the sentence. For example, the sentence "microglia migrated to the site of the lesion, which was associated with increased levels of ED-1" is somewhat vague, because it is unclear if the "which" is referring to the lesion or to the migration of microglia.

If there is ever any doubt about such a sentence, it is best to rephrase it completely. To avoid ambiguity, the sentence about microglia could be re-written as "migration of microglia to the site of the lesion was associated with increased levels of ED-1" or "microglia migrated to the site of the lesion, and immunohistochemical analysis revealed increased levels of ED-1 at this site."


  • "Data were normalised to the housekeeping gene actin, which was used as an internal reference..." Here the "which" refers to actin, which is therefore the subject of the following clause.
  • "Data were normalised to the internal reference housekeeping gene actin, revealing increases in the levels of..." To refer to the analyzed data in a subsequent clause, "which" would be inappropriate and introduce an ambiguity.