 Drag-and-drop is used when a user selects something in a window, then clicks on the selection and drags it into another window.Since the two windows may be handled by two different applications, these mechanisms require two different clients connected with the same X server to exchange data.The X Window System core protocol includes some requests and events that are specific to selection exchange, but the transfer is mainly done using event sending and window properties, which are not specific to selection transfer.