How to split sector?

[42PercentHealth]

Hi Doomworld, it's the mapping noob again. I have a situation where I drew a sector which is a narrow hallway, but then I put a door in the middle of it. Now the door is sector 223, but both sides of the door are sector 221. Is there a way to split these into 2 different sectors?

 

Using DB2, BTW.

 

Thanks!

[BigDickBzzrak]

press M and click on a part of the big sector

[42PercentHealth]

Thank you, @bzzrak! Just what I needed. Always wondered what that did, too... :-P