[canals-list] TV aerial experts?

[Roger Millin]

Can anyone tell me about horizontal and vertical polarisation of TV 
signals. When does a vertical polarisation occur, is it associated with 
a particular transmitter, how do you know what to use and when etc ?
Never run into the problem before but have seen an aerial that can be 
wound from horizontal to vertical so I'm interested to know.
Ta muchly.
Roger