invoice unquestionably amounted to an implied clear and unambiguous promise to accept on those terms. The doctrine can only be used as a ‘shield not a sword’ The full force application of the equitable maxim estoppel only allows a litigant to “use it as a shield and not as a sword” restricts the application of this doctrine to as far as only to provide a defence to a party and not to be used as a cause of action against another. In Combe v Combe, CA a husband promised to make maintenance payments to his separated wife but failed to do so. The wife brought an action to enforce the promise invoking promissory estoppel. The court held that promissory estoppel does not create a cause of action and as such the requirement of consideration in formation of contract is still relevant. Promissory estoppel is a rule of evidence that prevents the promisor from denying the truth of statement on which the promisee had relied. Denning LJ said: "The principle does not create new causes of action where none existed before. It only prevents a party from insisting on his strict legal rights when it would be unjust to allow him to enforce them". However, this requirement seemed changed in light of the decisions in Evenden v Guildford City AFC, here the courts held "that promissory estoppel can be a cause of action." Early cases indicated that there had to be a clear and unequivocal undertaking. However, this was overruled in Secretary of State for Employment v Globe Elastic Thread Co Ltd. The promise or representation must be “precise” and “unambiguous” although it does not mean that such promise or representation must be expressly made. The concept of ‘waiver’ has been recognised by both the common law and equity as a means by which certain rights can be suspended, but then revived by appropriate notice. In Hughes v Metropolitan Railway Co, It can be seen that this case did not involve a "promise" as such, but merely an "understanding". It had been suggested that Hughes could be restricted to "relief against forfeiture" cases, but this was specifically rejected in the later cases. This case was the one on which Denning placed considerable reliance in the High Trees case. It never applied to situations of part payment of debts, however, under modern law the concept of waiver has been effectively considered within