Polylines are very useful and can help drawings immensely. Unless someone decides to fiddle with the settings!
Today I found that someone had changed the default polyline width (i didn’t even know you could do that0# on drawing I was working on and also turned on show lineweights which made drawing details really difficult!
So if you want to turn the default polyline thickness back to “normal” or make it thin, change the PLINEWID variable to 0.