21 Parentheses

We are used to using parentheses to identify material that acts as an aside (such as this brief comment) or to add incidental information, but in technical writing the rules for using parentheses can be more nuanced.  Some more specialized functions of parentheses include:

• To introduce tables or figures within a sentence:

In pulse-jet collectors (Figure 3), bags are supported from a metal cage fastened onto a cell plate at the top of the collector.

• To represent converted units:

The funnel used for this experiment was 7 in. (17.8 cm) in length.

• When enumerating:

The system has three principal components: (1) a cleaning booth, (2) an air reservoir, and (3) an air spray manifold.

• To indicate product manufacturer names:

The filtering process involves a 10-mm Dorr-Oliver cyclone (Zefon International).

• To introduce an acronym after it has been written out:

Units will be expressed in cubic feet per minute (cfm).

Finally, it should be noted that punctuation used alongside parentheses needs to take into account their context. If the parentheses enclose a full sentence beginning with a capital letter, then the end punctuation for the sentence falls inside the parentheses. For example:

Typically, suppliers specify air to cloth ratios of 6:1 or higher. (However, ratios of 4:1 should be used for applications involving silica or feldspathic minerals.)

If the parentheses indicate a citation at the end of a sentence, then the sentence’s end punctuation comes after the parentheses are closed:

In a study comparing three different building types, respirable dust concentrations were significantly lower in the open-structure building (Hugh et al., 2005).

Finally, if the parentheses appear in the midst of a sentence (as in this example), then any necessary punctuation (such as the comma that appeared just a few words ago) is delayed until the parentheses are closed.