The English verbs discussed below all relate a whole thing to its parts. There is some confusion of this set of verbs by native speakers of English,* and there is also some controversy, so the correct meanings and proper usage of these words are worth considering.
These two verbs both declare what parts belong to a whole, with the grammatical subject being the whole and the objects being the parts. They are interchangeable.
The whole comprises its parts.
The whole consists of its parts.
Both verbs tend to specify a complete accounting or listing of the parts that make up the whole. If I say, for example, that my computer system comprises an iMac, a keyboard, a track pad, two external hard disk drives and external powered speakers, it means that there are no other components in my system. But if this is to be considered a 'rule', it must be a somewhat fuzzy one.
These verbs all indicate the subject is a collection of parts that are in the object (the whole). They are interchangeable in that meaning. The clear difference between this group of verbs and the others in this category is that these verbs take the parts as the grammatical subject and the whole as the object.
The parts constitute the whole.
The parts compose the whole.
The parts make up the whole.
Similarly to 'comprise' and 'consist of', these verbs specify the complete set of parts. If I say that an iMac and a keyboard constitute my computer system, it means that my system has only those two parts.
This verb also specifies the parts that belong to a whole. It has the same meaning as 'contain'. It differs from 'comprise' and 'consist of' in that it can specify either all or only some of the parts, although the latter meaning is perhaps most intended.
My system includes an external hard drive and an internal optical drive. (But there are also other components.)
Her system includes three laptop computers connected by Ethernet. (And nothing else.)
The biggest problem is confusing the sense of 'comprise' with the sense of 'constitute' or 'compose', particularly when comprise is incorrectly used in the passive voice to mean constituted by or consist of rather than the correct 'be composed of'. The confusion may have originated in the similarity of 'compose' and 'comprise'.
The legislative, judicial and executive branches comprise the US government. (Parts cannot comprise the whole.)
The US government is comprised of legislative, judicial and executive branches. (Making the verb 'comprise' passive does not change the relationship of parts to whole. The active form would be 'Legislative, judicial and executive branches comprise the US government.', which is clearly wrong.)
The improper uses described above are being seen and heard increasingly often, and examples of misuse can be found more and more often in published books and articles as well as in less formal writing found on the Web. The American Heritage Dictionary cites an example from Saul Bellow, "Put together the slaughterhouses, the steel mills, the freight yards ... that comprise the city". This trend in usage is reaching the point where it could be argued that the 'rule should be changed to fit the way people are actually using these terms. One reason to resist such change, if it comes from lack of education or laziness, is that the change disrupts the original logic or clearness in distinction, thus making the language harder to learn and use consistently.
The legislative, judicial and executive branches constitute the US government.
The US government comprises (consists, is composed of, or is constituted by) the legislative, judicial and executive branches.
The improper uses described above are being seen and heard increasingly often, and examples of misuse can be found more and more often in published books and articles as well as in less formal writing found on the Web. Acceptance of the misuse among the most highly literate of the US population also seems to be increasing. In 1965, 53% of the Usage Panel of the American Heritage Dictionary objected to the usage, but by 2011 only 33% objected. This trend in usage is reaching the point where it could be argued that the 'rule' should be changed to fit the way people are actually using these terms.
So, should we stand firm and call the misuse of 'comprise' and 'comprised of' straight-up errors and teach them as mistakes, hoping to turn the tide,win the day, and make the world a better place? Or should we be passive and accept the expanding misuse as a part of the natural evolution of a dynamic living language, no harm, no foul? To decide, I suggest that we consider what might be lost and what might be gained?.
"If it's not hurting anyone, don't be a dick."
Is anything lost by accepting the misuse of these terms? I say yes. One thing lost is consistency, and that's a pretty big thing, especially for the mish-mash, patchwork quilt of a language that we call English. Accepting 'to be comprised of' as the correct equivalent of 'to comprise' creates a huge semantic conflict, with the active and the passive forms of the same verb being synonyms. Would we accept that 'John ate the hummus.' and 'John was eaten by the hummus.' have the same meaning? By accepting a special case that does not fit a well-established pattern, we increase inconsistency and add to the already high difficulty of learning the language.
Another thing lost is simplicity. Creating a new term that adds no descriptive advantage for something already well described by an existing term just adds useless complexity. That is compounded when the new term is longer and grammatically more complex than the existing term.
What advantage comes with accepting the 'comprised of' usage? Nothing at all comes to my mind. If you can think of something on the plus side, let me know.
My conclusion is that we should certainly not accept 'to be comprised of' as a proper addition to or replacement of 'to comprise'. Doing so would add just one more to the myriad babysteps we are making in the march toward Idiocracy.
*This is true for American English at least; I'm not sure if this is also true for speakers of other English dialects, although a number of readers have suggested that it is.
American Heritage Dictionary definition with usage note
Frequency of 'comprised of' vs. 'composed of' in books
(Google ngram viewer)
Comprised of vs. composed of