Extension levers
In the early '70's, many people bought bicycles with drop handlebars, for reasons of fashion, even though drop bars did not suit their casual riding style. Given the frame and stem designs commonly available at the time, it was generally impossible to get drop handlebars high enough up to allow a low-intensity rider to reach the drops comfortably.
Dia Compe invented bolt-on extensions that allowed Weinmann-type brake levers to be operated from the tops and middle of the handlebars, making this type of bar bearable for casual cyclists, since they never had to use the drops. This was so popular that Weinmann traded licensing with Dia Compe, so that each could copy the other's products.

(Stem shifters were also popularized around the same time, and for the same reason.)

This system has several drawbacks:

The extension lever partially applied the main brake lever, reducing the available lever travel. Not all brands/models suffered from this, but the most common ones did.
The attachment hardware precluded the use of the top of the brake lever hood as a comfortable riding position.
They encouraged the practice of riding with the hands on the top, middle section of the bar, which is a position that doesn't give very secure control, especially on bumpy surfaces, because the hands are too close together.
The hardware that held the extension levers to the main levers was prone to fall off.
Other manufacturers produced similar systems, some of which addressed some of these difficulties.

Extenison levers are sometimes known as "safety levers." Since many people believe they actually reduce safety, the slang terms "death grips", "suicide levers" and "turkey wings" are occasionally substituted.

In the early 21st century, an greatly improved system of "interrupter brake levers " appeared, with all of the advantages and none of the drawbacks of the older extension levers. These also have the advantage of being compatible with modern "æro" brake levers which work a lot better than the older style levers that had the cables coming out of the tops.

Also known as inline or cross top brake levers.