Courvoisier Law states that an enlarged gallbladder which is painless is more likely to be due obstruction by carcinoma than by gall stones.
The idea is this,
Stone obstruction is usually acute and the gallbladder will contract in an attempt to override the obstruction and this will increase the intraluminal pressure causing the pain. (Charcot Triad = Fever + Pain + Jaundice)
While in carcinoma, whether it's cholangiocarcinoma or carcinoma of the head of the pancreas will cause a gradual obstruction and the gallbladder will not be so reactive and therefore these conditions will be painless.
However, in both the situations you have blocked the flow of bile and you have to have conjugated hyperbilirubinemia. Only one exception where you don't have jaundice, which is if the stone is lodged in the cystic duct, in this case you have pain but no jaundice because the bile can still flow from the hepatic ducts to the common bile ducts.
Pain + Jaundice ---> think gall stones
Pain + No Jaundice ---> think cystic duct stone
No Pain + Jaundice ---> think carcinoma