What is the criterion for distinguishing between a minor ('yaseer') and a major ('katheer') uncovering of the 'awrah?

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The limit of what is considered major is that which is deemed indecent or conspicuous upon viewing. There is no distinction between the two private parts (front and back) or other areas in this definition. The minor uncovering is that which is not indecent. The reference point for this distinction is custom ('urf). However, if the uncovered area pertains to the 'Mughallaza' (heavier impurity category), what is considered indecent might be less than what is considered indecent in other areas, and this difference must be considered in determining what prevents the validity of the prayer.