This car is celebrating its 100th birthday, and it would almost have been forgotten. If it hadn't given rise to an expression that we still use in everyday life today. Happy anniversary, Opel 4/12 PS!

Four-twelve? Even Opel fans won't understand it. The little classic is better known by the popular name "Laubfrosch" (Tree frog) due to its green colour. Opel introduced mass production with assembly line technology 100 years ago. The first car to roll off the production line in Germany in 1924 is the Opel 4/12 PS, which is always painted green. Incidentally, the numbers denote the rated horsepower (4) and the actual horsepower (initially 12). With the new model, Opel revolutionises the automotive industry in Germany.

Gallery: Opel 4/12 PS "Tree frog" (1924-1931)

The Opel 4/12 PS broke with convention right from the start in 1924. Instead of being large and black like most models, it was a small, robust vehicle in green - which quickly earned it the nickname "Tree frog". It had numerous innovations to offer, including a four-cylinder block engine with direct flange-mounted gearbox, removable cylinder covers, oil pump lubrication, steel multi-plate clutch, adjustable windscreen and steel disc wheels.

Like the wheels, the vehicle frame is also made of pressed steel; the rest of the body is made of steel and wood. Its one-litre in-line four-cylinder engine accelerates the small car to a top speed of 60 km/h and achieves an impressive 50 km/h in third gear. The gear lever of the right-hand drive vehicle is no longer mounted on the outside, but - very modern for the time - is easily accessible in the centre of the vehicle. Initially offered as a two-seater and later as an extended three-seater, the Laubfrosch is 3.20 metres long, 1.35 metres wide and 1.65 metres high, has a folding top as weather protection and even a boot integrated behind the seat bench in the pointed rear.

Opel 4/12 PS

Opel 4/12 PS "Laubfrosch" (1924-1931)

However, there was controversy about the frog: Citroën saw it as a plagiarism of its own 5CV. In lawsuits brought at the time, the claim was rejected by the courts due to several differences in vehicle details. In addition, six years after the end of the First World War, the French did not really have much of a voice in German courts. In addition, France had still occupied large areas of the Rhine and Ruhr in 1924, which led to massive conflicts. 

The most conspicuous outward change, however, was the green paintwork - Citroën painted the car lemon yellow. In 1924 and 1925, four times as many Opel 4 PS cars were sold in Germany as Citroën C models. Production of the Citroën 5CV was discontinued at the beginning of 1926. 

Opel itself put it this way: "The success of the first mass-produced automobile in Germany could not be foreseen when the Opel brothers decided in 1923 - in the middle of the inflationary period - to produce a vehicle using completely new manufacturing methods and thus revolutionise German automobile production. Shortly before this, Fritz von Opel had travelled to the USA to take a closer look at assembly line technology and the use of modern machine tools; he took further inspiration from France."

Opel 4/12 PS

The Opel brothers invest the enormous sum of around one million gold marks to modernise the plant and switch from individual to assembly line production. The first assembly line is 45 metres long - not much, but it revolutionises production. The conveyor chains that soon run through the factory are also new.

With them, the days when workers had to fetch their materials by hand or with a trolley were a thing of the past. Crankcases, camshafts and cylinder blocks are now transported on conveyor belts. The heart of the factory, the assembly line, also grew steadily: by 1928, assembly line production at the Rüsselsheim plant already stretched over a length of around two kilometres.

Opel 4/12 PS

Production on the assembly line reduces production times to a minimum. While Fritz von Opel hoped for a daily output of 25 vehicles in the spring of 1924, by the end of the year it was already 100 cars, and just a few months later 125 Leaf Frogs were coming off the production lines every day. A double advantage for the customers: Thanks to falling production costs as a result of increasing unit numbers, the favourable cost price of the Opel 4 PS continues to fall and the manufacturer is able to meet the continuously growing demand at the same time.

The "car for everyone", as Opel advertised at the time, cost 4,500 Rentenmark at the start of production - not much compared to other vehicles, but still as much as a home. It became the people's Opel because its sales price fell from year to year thanks to the continuous development of assembly line technology.

In 1930, the Opel 4/20 PS was available as a simple two-seater for as little as 1,990 Reichsmarks. With a good 120,000 units produced, it had already established itself as an ideal and affordable means of transport for commercial contemporaries such as country doctors, architects and sales representatives.