feature of the Kirby system is the modular design of both vacuum and accessories, allowing machines to be configured for numerous uses, from normal floor cleaning, tile surface scrubbing, wet floor mopping, to furniture shampooing and as a compact, handheld vacuum with extension hose attachments for various cleaning chores. This versatility also provides for add-on sales or bundling of the complete system. Starting in the late 1940s the carpet cleaning nozzle with the brush roller could be removed, which was powered by a removable rubber belt that was in contact with a metal shaft extending from the vacuum turbine blades, powered by the electric motor, and reattached to a brush used to polish floors, or an extended plastic hose that could accommodate a furniture dusting attachment. These attachments are still offered and the floor buffer was updated in the mid-1960s with a dry foam carpet shampoo attachment Kirby called the "Rug Renovator" that was first introduced with the two speed motor on the Dual Sanitronic 50 and is also still available. The carpet shampoo cleaning solution is in a clear tank that attaches to the motor exhaust where the dirt collection bag is attached as a vacuum cleaner, and uses compressed air generated by the electric motor. A plastic tube then sends the shampoo to the carpet scrubber brush that agitates the surface to be cleaned. Kirby has remained with its original design, materials and functionality with enhancements added to aid in its operation and durability. The company changes the appearance of the cleaner with revised color schemes and introduces new models with a similar core design. The attachments and appearance items are interchangeable between generations and some machines consist of parts from multiple models. Machines built in the 1930s, 40s, 50s and 60s are in operation worldwide and can be repaired or rebuilt with factory original parts. With the introduction of the Generation 3 model in 1990, all Kirbys are installed with a self-propelled transmission, called TechDrive Power Assist, which is engaged with a lever at the bottom next to the foot operated power switch. The Generation 3 also saw the introduction of a HEPA certified dirt containment filter bag, labeled as "Micron Magic" which minimizes dust being expelled into the air from the machine. In the 1930s and the 1940s, Kirby started to offer their products in retail environments, and introduced the "R" series. They are identical to