What does it mean to co-sign a letter of recommendation? 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 &quot;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.&quot;
My question is, what does it mean to &quot;co-sign&quot; 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?
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 &amp; [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.