A measuring gauge constructed of a transparent “L” shaped body having an elongated first arm and a second shorter arm disposed at 90 degrees to the first arm and defining a notch therein which forms a triangle with the first leg of the triangle angularly disposed with respect to the outer edge of the second arm. A plurality of indicia are carried by the two legs of the gauge. The indicia each include a center band sandwiched between outer bands with the inner and outer bands being of contrasting colors. The side edges of the gauge are inserted between the flaps and back panels of the envelope into engagement with a gum line and a depth of penetration is observed by determining the intersection of an edge of the envelope with the indicium along a side edge of the gauge thereby determining whether or not the envelope is in or out of specification.