Unity - each paragraph must deal with one topic or idea. Every sentence in the paragraph must be closely connected with the main topic of the paragraph. The topic sentence is one sentence of the paragraph that states the topic--generally the first sentence.
Order - there must a logical sequence of thought or development of the subject. All ideas should be connected with the leading idea and arranged according to their importance.
Variety - avoid monotony. The paragraphs in an essay should be of different lengths, and not always of the same sentence construction.