 The notice must be clear and unequivocal.Where the time for performance was not fixed in the contract itself, both a demand for performance ("interpellatio") and a notice of rescission are necessary to allow cancellation for non-performance, though both may be, and usually are, contained in the same document.The time element, for obvious reasons the most crucial element of "mora", depends on whether it is "mora ex re" or "mora ex persona".