*We measured the thickness of the bar before applying the solvent, and we measured the thickness after applying the solvent.

In this sentence, several elements are repeated in the second sentence: the subject, the verb, and the object, as well as the gerund applying the solvent following the preposition after. It is possible to think of these repeated elements as simplifying a basic algebraic expression 2a + 2b = 2 (a + b) -when the common elements between the two sentences are placed at the beginning of the sentence and apply equally to all subsequent elements. In this case, the second occurrences of we measured the thickness of the bar and applying the solvent would be extracted:

*We measured the thickness of the bar before applying the solvent, and after applying the solvent.

Then the and... after elements that are now left behind with nothing to conjoin should be moved forward to join the lonely before with and ... after ...:

We measured the thickness of the bar before and after applying the solvent.

The same approach can also be used to eliminate repeated elements of phrases as in the following examples:

• *can be measured and can be increased -> can be measured and ... increased
• *the metal spoon and the metal bowl -> the metal spoon and ... bowl
• *large factories, wholesalers, and big retailers -> large factories, wholesalers, and retailers

Another important point to make about using coordinate conjunctions is that they conjoin series of elements that have to be of the same type (e.g., nouns, noun phrases, or gerunds, verbs or verb phrases of the same tense and number, adjective or adjective phrases, or prepositional phrases). In effect, the term parallel structure refers to "a string" of elements that are similar to beads in a necklace: They can be a little bit different, but not dramatically so. Several examples from an authentic text on sources of the Western tradition are presented next.

Stoicism and its followers taught the universal principles or the natural law.
[Parallel noun phrases in the subject position: The first element of the subject contains a simple noun and the second a noun phrase. Also identical parallel noun phrases in the object position: Both consist of an article, an adjective, and a head noun.]

If the string of parallel elements consists of more than two phrases, commas are used to separate them, and the conjunction comes before the last item (this is how readers know that they have arrived at the last item in the string):

Stoicism gave expression to the universalism of the Hellenistic Age, and it held that Greeks, barbarians, senators, patricians, gladiators, or slaves were essentially equal because they all had capacity to reason.

In constructions with parallel noun (or more rarely) adjective clauses, clause markers must be retained (even when they are identical in form and function) because dependent clauses are marked as clauses by means of these subordinators. For example:

[Augustine] cautions the optimist that progress is not certain, that people, weak and ever prone to wickedness, are their own worst enemies, that success is illusory, and that misery is the essential human reality. (Perry et al., 2000, p. 192)

Such structures usually occur following reporting verbs (e.g., cautions, holds, mentions, notes, states), and, in general, in these and other contexts parallel clauses are not very common. In L2 prose, they may be particularly rare (Hinkel, 2002a).