TITLE: How are mathematicians taught to write with such an expository style?
QUESTION [77 upvotes]: I wasn't sure if this question was appropriate for MSE.
One of the major complaints we see in industry is a person's ability to communicate which includes writing.
We see the same thing on questions that are posted on MSE by younger students. 
However, the wonderful and skilled mathematicians on MSE both pose and answer questions with beautiful skill, directness, economy of words and all the rest. The grammar, flow and points are very clear and it is just beautiful expository writing.
In school, we were forced to take a class on technical writing, but this is not really the same thing. Even though I have a couple of different degrees, including mathematics, we were never required to take such a course.
My question is where and how do mathematicians learn this skill? Do some degree programs require a class on this or is this just learned by the school of hard knocks? I have even searched for books on the matter, but to no avail. Any insights would be appreciated.

REPLY [6 votes]: A course on writing proofs?  Yes, but on a really basic level.  E.g. Daniel Solow, How to Read and Do Proofs.  A student would see such a course when first starting to write proofs.  
Nowadays, Ph.D. theses in many places are written in English, and advanced instruction is in English, even if that is not the usual language outside the classroom.  Probably the professors work with students in these settings to improve their writing skills.  Similarly, in the US there are Ph.D. students from China, India, Bulgaria, Zambia, etc., etc., and the American professors work with them to improve their English writing style for mathematics.   
Advice for a student from China studying in the US ... get a professor who is NOT from China!