A boat or ship engaged in the tramp trade is one which does not have a fixed schedule or published ports of call. As opposed to freight liners, tramp ships trade on the spot market with no fixed schedule or itinerary/ports-of-call(s). A steamship engaged in the tramp trade is sometimes called a tramp steamer; the similar terms tramp freighter and tramper are also used. Chartering is done chiefly on London, New York, and Singapore shipbroking exchanges. The Baltic Exchange serves as a type of stock market index for the trade.