Regularity is a concept based on adherence to Masonic Landmarks, the basic membership requirements, tenets and rituals of the craft. Each Grand Lodge sets its own definition of what these landmarks are, and thus what is Regular and what is Irregular (and the definitions do not necessarily agree between Grand Lodges). Essentially, every Grand Lodge will hold that its landmarks (its requirements, tenets and rituals) are Regular, and judge other Grand Lodges based on those. If the differences are significant, one Grand Lodge may declare the other "Irregular" and withdraw or withhold recognition.
Why are landmarks for every Grand Lodge the same?