Compare and Contrast papers inform the reader about the differences and similarities among a set of two or more subjects, topics, events, locations, …etc. The writer must have a reason for creating the comparison and all of the characteristics compared and contrasted must revolve around that reason. The introduction is the perfect place to establish both the pattern and the basis of comparison. Whether a characteristic is relevant or not is dependent upon the reason for the comparison, the main point of the paper.
For example, if the writer is comparing a Granny Smith apple and a Golden Delicious apple as examples of variety within a genus (the overall category which apples fall into), they might mention the location in which each apple grows; the color, size, or shape; they might even mention the taste and uses of the apples. All of these characteristics are relevant to the main point of the essay (showing variety within the apple genus).
Characteristics that may be irrelevant to the essay would be observing where the sticker is placed on the apple, where the apples are distributed to, or how they are shipped. Although you could argue that these might eventually be relevant, a writer will not want his or her reader to have to work to find the relevance.
It is important to maintain the same basis for comparison for each of the subjects.
Back to our apples… The writer would not want to discuss the geographical properties of a Granny Smith apple to then only describe the appearance of a Golden Delicious apple. It is not an equal comparison. You could even say, it’s like comparing apples to oranges!
Remember: The overall goal of a comparison paper is to distinguish between two or more subjects, whether by focusing on their similarities or differences. This creates a foundation for discussion, for presenting the point of the comparison.