Flashing across the computer screen, we saw a startling message.

The program only reads PC files.

The DVDs displayed on the tables that are for sale seem to be selling well.

Researchers told reporters that they planned to close the lab after the mutated life forms escaped.

Swarming in dark clouds, farmers saw the locusts approaching their crops.

Hopefully, the zookeeper removed the bars between the male and female gorillas.

As a young man, his mother encouraged him to study science.

Leafing through the piles of test data, his eyes fell upon the very result he needed.

So far we've been dealing with fairly short sentences, and it's been easy to fix modification problems. All we've had to do is pay attention to getting modifier and thing modified in close proximity, and in some cases to insert a missing subject into a clause so that the modifier can clearly attach itself to the thing it's meant to modify.

What's that? Shouldn't this be enough, since good technical/professional writing is always composed of short, economical sentences?

No. Here's where we move on to a new level of stylistic sophistication. True, I've been encouraging you to write with economy, especially in the sense of using active voice, denominalizing, cutting fat, and reducing jargon. All this tends to reduce sentence length. But often, complex thought calls for a peppering of complex, longer sentences. The best writing is not an unending series of staccato sentences, page after page. The best writers mix their sentence lengths. Some are short, some longer, some quite long.

One common way to combine chunks of thought within a single sentence is to use restrictive and non-restrictive clauses (relative clauses) as modifiers. Let's examine that method, starting with fairly short sentences and progressing to longer ones.

Restrictive and Non-restrictive Clauses
A clause, you remember, is simply a phrase containing a subject (thing) and a predicate (something said about that thing). This is the formula for a simple sentence:

The robot won the competition.

Now let's say we want to modify the word "robot" by specifying which one, out of the hundreds entered, won the competition.

The robot that University of Tennessee engineering students built won the competition.

The clause "that University of Tennessee engineering students built" is actually a full sentence in its own right; it amounts to "University of Tennessee engineering students built [that robot]." But for our purposes of "sentence combining," we've chosen to nest one clause (sentence) inside another, allowing one to modify an element of the other.

The phrase "that University of Tennessee engineering students built" modifies the word "robot" in the basic sentence "The robot won the competition." Notice that it modifies in a special way: it singles out a particular robot from among the hundreds entered; it therefore limits or "restricts" the meaning of robot, so we call it a "restrictive clause."

Traditionally, we start such clauses with the word "that" or "who," although modern usage now allows "which." We omit punctuation at both ends of the clause; this is really the clearest sign we mean it to be restrictive. If we wish to fashion the restrictive clause without the "that" or "who" or "which," we can do so:

The robot University of Tennessee engineering students built won the competition.
Or,
The robot built by University of Tennessee engineering students won the competition. [Passive voice. This is just short for "The robot that was built by University of Tennessee engineering students won the competition," and it still creates a modifying, restrictive clause.

But what if the scenario is different? Let's say that only one robot was entered into the competition. It was a competition among various kinds of inventions, you see-new concepts in fishing poles, innovative eating utensils, etc. The main thing you're announcing is that the darned little robot won the competition! The fact that UT engineering students built the robot adds some interesting info about the robot, but it doesn't change the fundamental meaning of the sentence. Our sentence doesn't have to single out the UT robot from among other robots.

This scenario calls for a non-restrictive clause:

The robot, which was built by University of Tennessee engineering students, won the competition.
Or, using the modern option of "that" to begin a non-restrictive clause, we get:
The robot, that was built by University of Tennessee engineering students, won the competition.

I recommend sticking to the traditional signal of "which" to start off non-restrictive clauses. But either way, remember to punctuate each end of the clause with commas or parentheses or dashes. That signals the non-restrictiveness of the clause. At least, it does so among those who understand this convention of English.

Note: if the non-restrictive clause falls at the end of the sentence, the terminal punctuation is of course a period.

Let's take a moment to hone your mastery of restrictive and non-restrictive clauses, some of the most common modifiers in the English language.

Exercise 2
Identify the following sentences as containing either a restrictive or a non-restrictive clause:

We will offer the discount that will satisfy the customer.

We will offer the discount, which will satisfy the customer.

The agent, who was confused about our offer to his company, called again.

The agent who was confused about our offer to his company called again.

The certificates, which are essential to our decision, are stuck in the client's safe.

The certificates essential to our decision are stuck in the client's safe.

Now "toggle" the following sentences. If they contain restrictive clauses, change them to non-restrictive-and visa-versa.

This is the consultant, who wrote the security program. [=Non-restrictive.]

Our engineers are re-drawing the AutoCAD diagrams, which are too complicated for non-specialists to understand. [=Non-restrictive.]

Butterfly ballots that may yield ambiguous results should be re-counted. [=Restrictive]

So far so good. Restrictive and non-restrictive clauses make good modifiers--in moderation. The only problem is, it's easy to abuse them, just as it's easy to pile too many prepositional phrases into a sentence or to jam together too many nouns and adjectives. For the purposes of an amusing song or poem, this may be fine: "This is the woman who ate the pig that swallowed the frog that ate the fly," etc. But in professional writing, such clause stacking makes a very poor modification scheme:

A research team that tracks hydroflow that is not willing to work in the extreme weather conditions that prevail in Canada produces results that fall below the standards of teams that are willing to work anywhere, in any conditions that present themselves.

To improve this sentence, try transforming the first relative (restrictive) clause into an introductory subordinate clause, and don't hesitate to cut fat:

o When a hydroflow research team isn't willing to work in the extreme weather conditions that prevail in Canada, it can't match the results of teams willing to work anywhere, in any conditions.

Basically, readers don't like to process two or more relative clauses in a row, nor do they like to read more than one such clause attached to any one independent clause, unless you have a sophisticated prose architecture going on.

Exercise 3
Revise these sentences to avoid using strings of relative clauses. Remember, there is no single "right answer."

Bluegill are the most abundant species that eat these insects that inhabit the waters that lie south of the steam plant.

He designed a motherboard that gave birth to multiple daughterboards, which increased the cost of a computer that was already too expensive.

All conduit openings must have seals that exclude intrusion of any oil that may be forced into the tubes that feed fuel into the combustion chambers.