In case of a hung assembly, there are no set rules for the Governor. It depends on the Governor whom to call for forming the Government. Now the Governor has called Yeddyurappa as he earlier said that it will be 17nth for sworning in.
According to Harish Salve on the legal luminaries normally the party with the single largest majority is called and in case they fail then the next majority even if it is coalition will be called to prove the majority.
The Congress, on the other hand, is planning to go to the court in case the Governor calls the BJP to prove the majority. They quote the cases of Goa and Manipur where the BJP stiched the majority and did not give an opportunity for the Congress to prove the majority even though it was the single largest one.
But whoever is called the by the Governor the have to prove the majority within a weeks time. But Arun Jaitley former finance minister earlier tweeted that if there is a majority with the coalition of MLAs then the Governor should call this group. If it so then the Karnataka governor must call the coalition of Congress and JDS first as they have the numbers more than that of the BJP.
The communist leaders pointed out that the Governors in Meghalaya, Manipur and Goa did not call the leaders of the single largest party at all. So the confusion reigns supreme here in Karnataka. The Governor might take some more time and in the meantime, the BJP would get ready with the numbers and prove the majority on the floor of the house.