Shape is more indicative of function - i.e., what the bottle was used for or contained - but even that has a myriad of exceptions.
The bottle pictured to the right has a Wait's Liver and Kidney Bitters label pasted over the embossing of a Star Kidney and Liver Bitters bottle!
(The two products were from separate companies which were cross-town [Sacramento, CA.] rivals during the late 19th to early 20th century.
But any technique, once developed, can be used right up to the present - as many collectors know who have been so unfortunate as to rely too heavily on a popular termination date as sure evidence of true antiquity..." (Toulouse 1969b).
In short, there was (and is) nothing to stop a glassmaker from using an obsolete method in the production of a bottle.3.
Unfortunately, the complexities of precisely dating bottles is beyond the scope of any simple key.
A substantial amount of bottle type specific information must be reviewed by a user to increase the probability of dating accuracy.
As Berge (1980) noted in referring to bottles, the "..of manufacture of glass containers provides observable attributes which seem to be very useful in a classification of these artifacts." Thus, this page.
This bottle dating "key" is a relatively simple "first cut" on the dating of a bottle.
Additional reference materials outside of this website must often be consulted to narrow down the date of any item as far as is possible and to really get a "feel" for the history of the bottle in question.