To provide some background, I'm an MS student (CS - machine learning) looking to apply for PhD programs. I was looking for letter writers, and struggled to find the third.

I have the option of asking a postdoc that I've worked with or a professor who has explicitly stated that he "doesn't know me well enough to write me a detailed and strong letter, but if you have no one else I'd be glad to write one."

My question is, what does it mean to "co-sign" a letter? Many people have advised me that if a postdoc writes my letter then it would be a good idea to have our advisor co-sign it. As far as I know, universities send out requests to submit letters electronically and writers submit the letters online. My dad is also a professor (albeit not CS) and has said he hasn't heard about this ever since letters went online. What would this mean in today's context? Do some fields still request mail-in letters?

  • Most of this is just a copy of your earlier question. Pleas make this more concise. Make it specific to the new question.
    – Buffy
    Commented Nov 6, 2020 at 15:46

2 Answers 2


Co-signing something simply means that the co-signer also approves of the letter. For example if I wrote:

To whomever it may concern,

Seankala is a brilliant student.

Sincerely, Allure

If someone else were to co-sign it, it would become:

To whomever it may concern,

Seankala is a brilliant student.

Sincerely, Allure & [someone]

It means that that someone also thinks Seankala is a brilliant student.

Co-signing something doesn't mean one wrote or even helped write it. Here's an example of a co-signed petition. The petition itself was obviously not written by 12,000 people, but you can be sure they all approve of it.


"Co-signing" a letter has nothing to do with being a physical letter.

All it means is that the postdoc writes (at least most) of the letter, and then the professor (in theory) reads it, and also puts their name on it as a way of saying:

I am a qualified professional, and although I don't know the student personally, I support [postdoc]'s endorsement of them if they do.

The postdoc would probably still submit it in the system, and they would show up as the recommender, but if they opened and read the letter, they would see the professor's endorsement.

If both parties agree, it would generally improve your application.

Although I see that in your specific case, the co-signer is your advisor (who presumably already wrote you a letter). In that case, the benefit would seem to be slimmer.

  • When I was a TA, I cosigned rec letters for people who wanted recommendations from the professor of that course. In that case, the professor wrote the letter but quoted from some sentences I wrote about the student's class performance. So, same as your answer but flipped. The professor had a policy of not solo authoring these letters because they felt they didn't have enough personal interaction (unless they had other interaction outside the course).
    – Bryan Krause
    Commented Nov 6, 2020 at 16:03
  • 1
    A better formulation from the PI, would be "I am the supervising professor of this group and I agree with the things said." There is no need for the "don't know" clause". But getting the PI to agree might be an issue. The post-doc should be the one to suggest it. (For "supervising professor..." substitute whatever is honest. The professor would be just agreeing that the opinions of this post-doc have validity that they recognize and hope others do also.
    – Buffy
    Commented Nov 6, 2020 at 16:04
  • @Buffy Ah, the quote wasn't meant to be an extract from the letter, it was meant to be the subtext of what's being said. While I agree the prof doesn't need to say they don't know the student that well, that's what is understood from them cosigning a letter written by their supervisee. Commented Nov 6, 2020 at 16:11
  • Would the benefits reduce to the point that the postdoc's letter would be less valuable than a lukewarm letter from the professor?
    – Sean
    Commented Nov 6, 2020 at 23:27
  • 1
    @Seankala Hard to say. Depends on how good the postdoc's letter would be. I was probably recommend the professor over a letter from a postdoc in your group countersigned by someone who already endorsed y ou. Commented Nov 6, 2020 at 23:49

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