Düsseldorf mustards are somewhat like Dijon Mustard, but tend to be somewhat more pungent than Dijon, being made with vinegar instead of verjuice, and darker.
The flavour can range from milder, to hotter than Dijon, to quite hot, depending on the mustard seed used. Either brown and white mustard seed, or decorticated black mustard seed might be used. Some kinds may decorticate the brown mustard seed as well. There may also be a very small amount of ground clove and ground black pepper in the mixture. 
Dusseldorf Mustards won't be as sweet as Bavarian mustards; instead, they will have a bit of a sweet and sour taste.
Well-known brands include Koops, and Löwensenf (which means Lion's mustard.) Löwensenf has dropped the term "Düsseldorf mustard" from its labelling, aiming instead for a more global market. 
The oldest brand of Dusseldorf Mustard may be ABB (Adam Bernhard Bergrath), made since 1726. It is made from brown and yellow mustard seed, using brandy vinegar ("Branntweinessig".)  It is sold in grey earthenware pots in higher-end grocery stores. (It is also sold in 10kg plastic pails for the catering trade.)
The Löwensenf company was founded in Metz, Lorraine, France by Otto & Frieda Frenzel in November 1903.  They moved their business to Düsseldorf in 1920. They introduced into Germany machines that could decorticate mustard seeds. 
Löwensenf acquired ABB sometime in the 1960s.
In 1877, Heinz added mustard its product line. In fact, they added two versions of mustard: yellow, and a brown, sweet-and-sour version they called "Dusseldorf mustard." Owing to its popularity with German immigrants to America, Heinz was able to sell it for twice the price of their yellow mustard. 
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-- George Edward Bateman Saintsbury (English writer. 23 October 1845 - 28 January 1933)