Orrefors is a Swedish art glass company that ramped up its production in 1916. It began using numbering systems on the bottom of its pieces in 1917. Each system enables a collector to date a piece of Orrefors art glass by looking at the engraving on the bottom. The earliest pieces were engraved with a letter --- representing a designer's name --- and a number. Over time, Orrefors changed its numbering system; some pieces were engraved with a letter to represent a designer, followed by another letter to represent the glass type, followed by a pattern number.
Look at the bottom of the art-glass piece: "Orrefors" should be engraved on the bottom in cursive, along with letter and number identifiers.
Note the letter identifier. If the letters "G," "H" or "L" appear first, followed only by a number, then the piece was produced between 1917 and 1931.
Look at the number identifier. Pieces from this period are numbered from 35 to 948. The lower the number, the older the piece. For example, if you see "G 40" or "H 35," this indicates that the piece dates to 1917 or 1918; if you see "H 948," the piece dates to 1931.
Check the bottom of the piece: "Orrefors" should be engraved in cursive.
Look at the identifiers. The presence of two letters indicates Orrefors' second numbering system, meaning that the piece dates from 1926 to 1934. The first letter represents the designer, and the second represents the glass type.
Note the pattern number. Depending on the glass type, a lower number usually indicates an older piece. For example, "GE 11" would date to 1926. Although some glass types bear a much higher pattern number, they still date to roughly the same period. For instance, "GA 234" would date to 1929.
Make sure "Orrefors" is engraved on the bottom of the piece.
Look at the letter identifier. The presence of only one letter --- A, E, U or Z --- indicates glass type and typically means the piece dates anywhere from 1935 to 1960.
Note the number. The numbers used in the third numbering system range from 1400 to 3832. A lower number indicates an earlier production date. For example, "A 1400" would date a piece to 1935; "E 3832" would date a piece to 1960.
The best way to date art glass produced by Orrefors is by looking at its catalogues. The original catalogues have been referenced and/or reproduced in a number of books on Swedish art glass. Often, these sources include illustrations and pricing guides.