Writing letters of recommendation for more than one student to the same department Suppose multiple students have asked me for letters of recommendation to the same graduate department in the US. Am I expected to explicitly compare the students to one another? Am I supposed to do this in both letters (assuming they haven't been submitted yet)? Do I need to explicitly state a preference for one over the other, or can I just get away with listing relative strengths and weaknesses (for instance, "X is stronger academically, but Y has more research experience and is a stronger programmer")?
Since each individual student is asking for a letter of recommendation it would be appropriate to approach each letter as an individual task and not implicitly cross-reference between the letters by making comparisons. Direct comparisons between the specific students should not be made. It is after all the person or committee admitting the students to the program that will make decisions who may fit the best and their criteria may not be the same as yours. It is, however, reasonable to make implicit comparisons such as ranking each in relative terms to, say, all students you have encountered or some other frame of reference. This way each student is compared to a group and not each other. This is what one commonly does anyway.

I can imagine that you may get a request for more details by the person/committee if there is a difficulty in separating students. You could if you are willing, add a comment to the fact that you are willing to answer any questions that may arise during the process regarding the student in question.