A seam in old glassware means that the piece was made with a mold. Molten glass was injected into a mold to solidify. Molds came in pieces and were clamped together to make the glass, bottle, or whatever. When the mold was removed, a trace was left where the pieces of the mold met. Some manufacturers polished the seams off so that you could not see them. Lower quality products still have the seams. But just because there is a seam does not mean the product is not valuable today. Many types of older glassware produced for the lower classes can be highly collectible and very popular.