Hi Keith, I will add some info here....
Tango, by definition is actually two colors.... A base color of glass, accented most commonly with Black or Blue, depending on the base color. Technically this piece would not be termed Tango. The term Tango is now greatly overused and abused, mostly on the internet, but it's abuse and overuse has also, unfortunately, leaked into venues other than the internet.
The development of Tango is attributed to Loetz, although not all two color products they produced would qualify as Tango. For example, Ausfuhrung 166 and 216 (see Loetz.com) are commonly referred to as Tango, but are technically not that decor.... The appearance of Tango designs by Michael Powolny in 1914 exerted a strong design influence on the company's production and many similar designs developed as offshoots of his limited work are now mistakenly attributed to him. This, unfortunately, includes much glass not even made by Loetz and in no way connected to Michael Powolny.
Although Loetz is credited with developing Tango as a defined decor, the largest producer of commercial Tango style glass was Kralik. Many other glass houses also produced Tango style product during the height of it's popularity.
The book which Jindrich mentioned, Tango Glass by Herman Spaink is a book which not only contains false attributions, but is supposed to be based on a collection in a Museum which in fact does not exist. It also attributes many pieces pictured in the book to a company referred to as Loetz Kralik, which is a fantasy company that in fact never existed. It is the condensation of two well known firms into one company name. Kind of like calling a car something made by Ford Chevrolet, or a Mercedes Porsche. To err on the side of safety would be to totally ignore any text information contained in the book, and enjoy the pictures, which are actually well done.
There is a review of the book here:
http://www.glassmessages.com/index.php/topic,28220.0.htmlHope this helps,
Craig