Unity means that everything in your essay must be related to and in support of the thesis statement. The introduction should lead into the thesis statement, and the conclusion should lead out of it; each topic sentence should directly support the thesis statement, proving it to be true; every major supporting detail should directly support the thesis statement through its topic sentence; in fact, every sentence in the essay should directly support the thesis statement in some way.
One way of thinking about your essay is that the thesis statement is a summary of the whole essay; therefore, everything in the essay must be "covered" by the thesis in come way. Similarly, topic sentences are like summaries of whole paragraphs, and major supporting details are like summaries of the example or explanation which they introduce.
It is easy to make unity errors by "going off on a tangent"; that means to write about a related point, usually within the same topic, but which does not support your thesis statement directly.
For example, your thesis statement may be about how your friend is lazy; at one point, you might find yourself writing about how the friend often forgets things. This is a mild tangent: forgetting things does not strongly support the idea of laziness. It seems similar, because you might associate things like lateness to laziness, and forgetting appointments also makes your friend late. However, forgetfulness is not really laziness.
At another time, you might start writing about how your friend was foolish. This is a greater tangent: you might think they are related because both are examples of bad character. However, being foolish is quite different from being lazy.
You must always think about whether the sentence you are writing is unified with the rest of the essay or not.
An example of poor unity in topic sentences:
- Thesis statement: My sister is clumsy
- She always falls down
- She always says the wrong thing
- She always wears the wrong fashions
The last point seems connected because it is an error similar to the other items. However, the thesis is about being clumsy, and wearing the wrong fashion is not an example of clumsiness.