Coherence in a paragraph is the technique of making words, phrases, and sentences move smoothly and logically from one to the other. In other words, the ideas are so interwoven and "glued" together that the reader will be able to see the consistent relationship between them.

It is obvious that if a paragraph is not unified, does not have a logical order, and does not have a consistent point of view, the reader is unlikely to grasp the point of the paragraph. In addition, there are other devices and techniques that will help you achieve coherence.

In general, the coherence devices most helpful for making your communication clear for the reader are transitional words and phrases, repetition of key words and phrases, pronoun reference, and parallel sentence structure.

TRANSITIONAL WORDS AND PHRASES
One of the most common methods of establishing coherence in a paragraph is the use of transitional words and phrases. These devices indicate to the reader the specific relationship between what was said and what will be said.

Coordinate Conjunctions
Within a sentence, the coordinate conjunctions provide a coherent link to indicate relationships between parallel elements. The word and indicates a comparable idea; but and yet indicate a contrasting idea; or and nor indicate an alternative idea; for indicates a reason for a result already stated; and so indicates a conclusion from reasons already stated.

Correlative Conjunctions
Also within a sentence, correlative conjunctions are used to establish relationships between parallel elements. The conjunctions either/or, neither/nor, and whether/or (not) are used to indicate parallel alternatives. The conjunctions not only/but (also) and both/and indicate parallel similarities.

Words and Phrases That Indicate Specific Relationships

        To signify an additional idea - also, moreover, again, further, furthermore, in addition,
                    likewise, too, first, initially, second, third, next, finally, last, another, other, then

        To signify a comparison - likewise, similarly, in a like manner, in comparison, so it is

        To signify a contrast - however, nevertheless, still unlike, in contrast, conversely, on the
                    contrary, on the other hand, whereas

        To signify a cause-effect relationship - therefore, thus, hence, then, consequently,
                    accordingly, in conclusion, as a result

        To signify an example or specific meaning - for example, for instance, to illustrate, for
                    one thing, frequently, in general, in particular, namely, usually, specifically

        To signify a summary - to sum up, to summarize, in short, to conclude, in conclusion, on
                    the whole, in brief.

TRAIN OF THOUGHT

A full sentence is not always a full idea. It usually takes more than one sentence to develop a train of thought. Links in a train of thought include such words and phrases as conjunctions (and, but, so, therefore, because, once, still), pronouns (he, they, both), demonstrative adjectives (this, that), adverbs showing how things are related in time (once, then, later), in space (upward, downward, around), or as cause and result (as, as a result, because of), and synonyms that rename. Sentences may be linked in a train of thought by many words and phrases such as those listed below.

    also
    as a result
    as well
    at first
    at last
    earlier
    finally
    first, second
    for example
    further
    however
    in addition
    indeed
    in fact
    in short
    instead
    later
    likewise
    more
    moreover
    once
    on the contrary
    on the other hand
    other
    rather
    similar
    so much for
    such
    that
    that is
    then
    therefore
    these
    this
    those
    to sum up
    what's more
 

• A train of thought can be derailed by a shift of verb tenses. Once a verb tense has been established in your essay, keep all verbs in that tense unless there is some clear reason for using more than one tense. When you must shift tenses, follow a clear and appropriate sequence of tenses.
• A train of thought can be derailed by a pronoun whose reference is not immediately clear.

• A train of thought can be derailed by a shift from first person to third, from third to second, from second to first, and so on. Any pronoun that switches from the person already established in a passage should be replaced by a pronoun that maintains consistent person.
• Simple repetition is one of the most effective ways of keeping your reader's mind on a train of thought. Don't avoid repetition, but use it carefully. Use it when it really improves your expression and helps readers to understand your point.
• Parallelism, or repetition of grammatical and structural forms, can help you make clear the similarities among ideas and thus clarify your train of thought.
• The haste of writing a first draft may cause you to start on one train of thought but finish on another. The result will not clearly say what you mean.
• Confusing sentences often result from (1) leaving out essential words, (2) awkwardly repeating words, and (3) neglecting to complete a structure.
• In the haste of a first draft you may write contradictions and absurdities. Read your work with a critical eye, trying to look at it as a reader does. Revise to eliminate the contradictory, the absurd, and the illogical.