Harry Bertoia (1915-1978) moved to Pennsylvania from California to work for Knoll in 1950. Knoll is the manufacturer and copyright holder of all Bertoia chairs. Chairs, based on steel grid wire forms, were designed by Harry Bertoia between 1950 and 1951 and further developed for production by Harry and the development team consisting of Richard Schultz, Don Pettit and Bob Savage in 1951 – 1952. They were first introduced in December 1952, and have enjoyed steady continuous production since 1953. To state the obvious, nearly all products that have been in production for over 63 years undergo evolutionary changes over time. Most are a response to field failures, structural improvement or cost reduction. Others are a response to external forces. For the sake of discussion, one can consider all Knoll Bertoia chairs as being either early style (1986 and earlier) or late style (1986 and later). Prior to 1986, all Bertoia chairs were manufactured in East Greenville, Pennsylvania, or overseas by Knoll subsidiaries or under license. At various times, Bertoia chairs were manufactured in Argentina, Belgium, Finland, France, Germany, Italy, Japan, Sweden and Switzerland. All chairs of pre-’86 manufacture are very similar and are often only distinguishable by the wire diameters used (close metric diameter equivalent to US standard wire gauges.) See Photo 1 with yellow tags. It must be noted that, since foreign-made Bertoia chairs were never imported to the US, that it is very unlikely to encounter them here in the US. All chairs made before 1986 are characterized by lightness and transparency. Harry liked to think of his chairs as being “mainly made of air.” That changed when the manufacture of the entire Bertoia line was shifted to Italy. At that time, the collection was retooled and major changes were made. Most obvious was the significant increase in wire diameter used throughout which changed the character of the chairs from transparency to form. Other changes include altering the seat-back angle, the addition of a “waterfall” at the front edge and the use of the “paperclip-type” base connectors. Many other minor changes have taken place over the years. Most of the comments below are specific to the 420 Side Chair but apply to other models as well. These include: GLIDES – Various glides have been used over the years… or no glides at all. The first ones were extrusions that snapped on (and unfortunately snapped off). Later ones are injection-molded with an integral pin that is driven into a blind hole on the bottom of the base. “KNOLL” STAMP – In 2004, Knoll began to discreetly stamp the word “Knoll” on the upper part of the base. UPHOLSTERY – All chairs are available with or without upholstery, either in the form of a seat pad or fully upholstered. LINE ADDITIONS – During the 1950s and 60s, a number of additions were made to the original scope. These included the 427 fiberglass version of the 420 shell, the 425 (child-size 420 chair), the 426 (baby-size version of the 425, and the 428 barstool (in two heights). During the 2000s, two more designs were added. The Asymmetric Chaise, which was part of the original scope but never produced, was finally put into production in 2005. In addition, a new scaled-down child’s version of the 421 small diamond chair (never part of the original scope) was also introduced. Finally, an injection-molded polypropylene version of the long discontinued 427 shell (side chair) was reintroduced in 2016.