What exactly are flannel trousers?

[OwlHoot]

I've been invited to an event in a couple of weeks, and I'm told the dress code includes flannel trousers.

Although I've heard of them, I've no idea what they look like or where to buy a pair. Any ideas?

[NumptyCorner]

[DimPrawn]

Are they for incontinence? Just stick the wifes face flannel down your trousers incase of any embarrasing accidents.