Both of these words have something to do about the parts in a whole, but they have entirely different definitions. The issue is further complicated by variations on the usage of these words, like “is comprised of” and “is composed of.” Which formation is correct?
First, let’s start with the basic definitions. To comprise means to contain or to include. By contrast, to compose means to make (by combining things), to put in order, or to be a part of. In this way, the differentiation between the two terms in this context is relatively clear:
The whole comprises the parts.
The parts compose the whole.
This is when the verbs are used in their most basic form and in the most basic structure. For example, the associated pair of sentences could be:
My strategy for improving my posture comprises four techniques.
Four techniques compose my strategy for improving my posture.
Do you see the difference? Where there appears to be a greater confusion is when you use the construction “to comprise of” or “to compose of,” because only one of these is actually correct. While “to comprise of” is being increasingly used by the masses, it is technically incorrect. It would be like say, “my grocery list is contained of apples, oranges, and pears.”
Instead, that same sentence can be constructed in two alternative ways, both of which are acceptable and correct:
My grocery list comprises apples, oranges, and pears.
My grocery list is composed of apples, oranges, and pears.
The whole is composed of the parts. If the proper usage of “comprise” still confuses you, the easiest solution is to avoid the issue altogether and not to use “comprise” at all. Instead, consider alternative words and phrases. My grocery list includes apples, oranges and pears. My grocery list is made up of apples, oranges, and pears. There are countless other ways of saying the same thing.
Issues surrounding “comprise” and “compose” are nowhere near as common as those surrounding affect and effect, but that doesn’t mean that you should be any less diligent. Proper grammar is important and it is worthy of a little more effort on your part to ensure that you are using the right words under the right contexts.