Difference: 帘 vs. 帷? Came across the character 帷 for the first time the other day, in the word 窗帷.

Oxford, KEY and Adstrotrans define 帷, simply, as:


  curtain


ABC and A Chinese-English Dictionary define it as a:


  bed-curtain


which doesn't really mean much to me.

What's the difference between 帷 and 帘?
I'll assume you mean 帘 as in the simplified Chinese for 簾. In which case, strictly speaking:


帘 / 簾 is a cover for windows woven from bamboo slips (hence the traditional top radical) or fabric 
帷 is curtains, in the sense of sides a tent woven from fabrics.


In practice, they both mean some sort of fabric material that obscures sight, and therefore usages have become mixed up. In this case, the 窗 in the word 窗帷 specifies its a window curtain, making it the same as 帘 / 簾.



Traditionally, 帘 is actually a different character from 簾. It meant "a banner (of an inn or tavern)", and was flown outside such establishments.


  《唐·鄭谷·旅寓洛南村舍》
  
  　村落清明近，秋千稚女誇。春陰妨柳絮，月黑見梨花。
  
  　白鳥窺魚網，青帘認酒家。幽棲雖自適，交友在京華。 


Because 帘 also happens to be the simplified Chinese character for 簾, this difference is not noticeable in a majority of online resources anymore.



Historically, 簾 is what court officials use while 帷 is what common folks use. This is Ancient China historical.


  《禮緯》 天子外屛，諸侯內屛，大夫以簾，士以帷