Why are there little ribs on the sides near the bottom of the inside of some mugs?

little mugs near ribs
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Why are there little ribs on the sides near the bottom of the inside of some mugs?

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These ribs were added to the earliest glass mugs around the early 1950s to hold the soap in place. As the soap wore down from use, it could spin in the bottom of the mug while working up lather with a brush.