Giving a bottle an arched shape at the bottom means that if it does sag, it can do so without touching the bottom.(British Glass 2004) In conjunction with the finish (lip), the various attributes and features found on the base of a bottle allows for some of the better opportunities for the manufacturing based dating of a bottle.These are usually some type of embossed letters, symbols, and/or numbers which were molded onto the base of the bottle for a variety of glassmaker determined reasons.
For more information on the subject of glassmaker markings see the Makers Marks page, which is a sub-page linked to the Glassmaking & Glassmakers page.This page is divided into two sections based on the primary methods by which bottles were manufactured - mouth-blown (hand-made) or machine-made (both semi and fully automatic) - since base features on these types of bottles generally differ significantly.When referring to "markings" or "features" on the base of a bottle on this website, we are referring to embossing, mold lines, attached glass and/or mineral deposits which are confined within or around the edge of the bottle base.(See the Bottle Morphology page for more information on the major parts of a bottle.) covered in any depth on this page is the base profile or shape.The bottom of a bottle is usually the thickest part, retaining more temperature throughout the production line.
Because the bottom is hotter, it is also more fluid and has a tendency to sag, forming a shape like a spinning top which makes it unstable on flat surfaces.
For example, overall the three bottles in the picture at the top of the page are (from left to right) - octagonal, rectangular, and square.
However, the rectangular and square bottles also have distinctly beveled edges and the octagonal bottle is actual round right at the heel of the bottle - all of which complicate describing the base profiles a bit.
This is because there is little dating information to be gleaned from simply the base profile, though the profile can assist at times with the typing of a bottle.
Simply put, this is the shape of the bottle base when viewed straight on and is usually the cross-section shape of the bottle body, though there are exceptions to the latter.
Consult the IMACS website above if interested in the nomenclature of describing base profiles.