Is there any difference between 姨丈 and 姨夫？ My Chinese friends taught me to refer to the husband of my maternal aunt as 姨丈。My friends are perfectly familiar with my family and how we are related to each other. Specifically, the uncle in question is the husband of my mother's youngest sister.

Today my (university) teacher (who only speaks Chinese and therefore we can't easily communicate on nuances) corrected all occurrences of 姨丈 to 姨夫。The dictionary treats them as synonyms. Is there any difference? 

My teacher is from Beijing. My friends are from 浙江。I believe that my friends from 江蘇 and 台灣 also use 姨丈，but I might be mistaken on that.
amongst "姨丈", "姨夫", "姨父" or "姨爹"; "姨丈" is the most common one.

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000150395

imo, your teacher is . . . :(