What's the difference?

Can anyone tell me the difference between 枝 and 支? Do I write 一支笔 Or 一枝笔

http://www.hwjyw.com/teacher_hua/200910/t20091009_32434.shtml Unless you are talking about real twigs and sticks, it's better to use 支. Even stick shaped, non-stick items should take 支 with guns as an example in my Perapera-kun dictionary.