Symbols
Non-polarized: 



Polarized: 




Polarized, dual with common cathode, many variations: 


Variable/trimmer: 


Preset trimmer: 


Ganged trimmer, many variations: 


Feedthrough: 
These symbols are a mix of American, European and even Japanese.  Some are old and you may never see them anywhere else.  There are many variations on these but you can mostly puzzle them out.