[pirate22]

With sayc and sayc2/1-come to that, any system.
have noticed 7-or 8-or 9 of any suit other than n/t
and to some extent unbid suits def suit contract,what happened to 4th
highest leads----how is ones partner to defend,when pard leads any of the
above-- a total mystery to me,as a declarer i always get an extra trick from it

[manudude03]

Rule of 11 helps a lot (assuming 4th best).

Subtract the lead from 11 and it says how many higher cards are in the other 3 hands. Since declarer and 3rd seat can see 2 of these 3 hands, they can work out something about the other.
When it comes to high spot cards, a common agreement is to lead 2nd from bad suits. So if after using the rule of 11, you find that declarer only has 1 higher card (or none), work out if it is feasible from the auction. If it is not, then assume it is from a bad suit. You generally only want your side to only play 1 high card for each of declarer's honours.