I'm rebuilding this from the ground up as I can only remember the theme and the punchline.
A groom and his best man were called upon to sign a document after the wedding service.
The groom, who had never learnt to write, inscribed a large 'X'.
The best man, at his turn, wrote 'XX'.
'What's the second X for?' asked the groom.
'Oh, that's my economics degree.'