The model year of a product is a number used worldwide, but with a high level of prominence in North America, to describe approximately when a product was produced, and it usually indicates the coinciding base specification (design revision number) of that product.
The model year and the actual calendar year of production do not always coincide. For example, a 2009 model year automobile is available during most of the 2009 calendar year, but is usually also available from the third quarter of 2008 because production of the 2009 model began in July and August 2008. When a brand new model is introduced there may be an additional delay to retool and retrain for production of the new model.
The variables of build date and design revision number are semi-independent. There is no natural law that forces one to be strictly correlated to the other, other than the two facts that (1) future design revisions cannot have been built in the past, and (2) most products, in most contexts, tend to be built to the design revision that was the latest one at the time of building. The idea of strongly correlating build year to design revision number took many years to become a meme in consumer culture.
Alfred P. Sloan extended the idea of yearly fashion change from clothing to automobiles in the 1920s. His firm General Motors was the first to systematize the process of slightly altering cars every year to grab the buyers' attention.
The term may also be used by European and Japanese automakers in respect of model availability dates in North American markets: these often receive updated models significantly later than domestic markets, especially in the event of unforeseen slow sales causing an inventory build up of earlier versions.
The practice of identifying revisions of automobiles by their "model year" is strongest in the United States. Typically, complete vehicle redesigns of longstanding models occur in cycles of at least five years, with one or two "facelifts" during the model cycle, and are introduced at various times throughout the year. Additionally, introductions of new models are often phased in around the world, meaning that a "2004 model" of a particular vehicle may actually refer to two entirely different vehicles in different countries. Therefore, the more common practice for enthusiasts and motoring writers in other countries is to identify major revisions using the manufacturer's identifier for each revision. For instance, the Holden Commodore, a popular Australian car, are grouped into the following series: VB (introduced 1978), VC (1980), VH (1981), VK (1984), VL (1986), VN (1988), VP (1991), VR (1993), VS (1995), VT (1997), VX (2000), VY (2002), VZ (2004), VE (2006) and VF (2013). This is done for the simple reason of making the cars more easily distinguished.
In the automotive industry the "model year" is absolutely defined only by the manufacturer, and not by any local vehicle registration practices or marketing opinions.
Industry practice varies between markets according both to the level of exports to North America, and the extent to which US owned subsidiaries dominate the domestic automarket. In the 1960s and 1970s, many new models were traditionally introduced at the London or Paris motor shows during October, and manufacturers owned by US corporations as well as domestically controlled UK auto makers tended to follow US auto-industry conventions in respect of model years. The concept was never so universally applied in Europe as in North America, however, and since the 1980s, the more commercially critical European Motor Shows have been the March Geneva Motor Show and the September Frankfurt Motor Show, new models have increasingly been launched in June or July even in the UK, where the two remaining US owned subsidiaries no longer design and build distinctively British Ford and Vauxhall models. All this has left the US style model year concept increasingly absent from the European domestic automarkets.
An automotive model year is categorically defined by the 10th digit of the Vehicle Identification Number (VIN), and simply indicates any manufacturer-specified evolution in mid-cycle of a model range - such as revised paint options, trim options or any other minor specification change. The 10th VIN digit does not relate to the calendar year which the car is built, although the two may coincide. For example, a vehicle produced between July 2006 and June 2007 may have a 7 as the 10th digit of the VIN, and another vehicle produced between July 2007 and June 2008 may have an 8 in the 10th digit - with the change-over date varying depending on manufacturer, model and year.
In the United States, automobile model year sales traditionally begin with the fourth quarter of the preceding year. So model year refers to the "sales" model year; for example, vehicles sold during the period from October 1 to December 30 of the following year are considered one model year. In addition, the launch of the new model year has long been coordinated to the launch of the traditional new television season (as defined by A.C. Nielsen) in late September, because of the heavy dependence between television to offer products from automakers to advertise, and the car companies to launch their new models at a high-profile time of year.
In other cases, products of a previous model year can continue production, especially if a newer model hasn't yet been released. In that case, the model year remains the same until a new model is introduced. This is to ensure that the model will be seen by the public, and will actually sell an amount of vehicles before a new vehicle model is produced, and people will look at the newer model rather than the previous one.
In the United States, for regulation purposes, government authorities allow cars of a given model year to be sold starting on January 2 of the previous calendar year. For example, this means that a 2013 model year vehicle can legally go on sale on January 2, 2012. This has resulted in a few cars in the next model year being introduced in advertisements during the NFL's Super Bowl.