Dating Bottles with the The concept is that the higher the side mold seam on the bottle the later it was made - at least in the era from the early to mid 19th century until the first few decades of the 20th century.

Why does glass have a seam?

Molded or pressed glass will not have a pontil mark on the bottom. Instead, look for seams present to indicate that a mold was used in the manufacture, as mentioned above. The mold seams are usually found on the sides of the piece where a mold would have fit together during production.