Consisting of four Tau Crosses, this form of cross was assigned to the knights of the Teutonic (Germanic) Order, founded by Emperor Henry VI as a hospital order in 1191, similar to the Templars and Hospitallers.
By 1198, they became a religious military order of crusaders, first fighting in the Holy Land and later in Prussia and Lithuania. The order secured high status for themselves, but this ebbed in the 15th century. It was finally broken up by Napoleon in 1809 who gave the order's main assets to his allies of the Confederation of the Rhine.
Their founder, Henry VI, was physically not a very imposing character, but he made up for this with his strong diplomatic skills - and a strong army. In addition to being the Holy Roman Emperor, Henry was King of Germany and was later crowned King of Sicily. It was from here that 60,000 soldiers rode off as knights to join the Third Crusade.
Like today's crusades in the Middle East, there was a higher objective than merely spreading an ideology; it was about empire-building, acquiring as much property and power as possible. Not content with the better part of Europe, the king wanted more. (...Moors?)
The knights didn't spend all their time killing people however; they also guarded Christian pilgrims who visited a tented hospital at the city of Acre. From this, the Teutonic Order was established, and the knights worked alongside the other two Jerusalem orders - the Templars and the Hospitallers. Pope Innocent III granted the Teutonic Knights the honour of wearing a white habit with a black cross.
The knights conquered Acre, which probably seemed like a good idea at the time, but here we see yet another example of the futility of war, since the Muslims reclaimed the city about 100 years later.
In the 13th century, they turned their attention to attacking pagans in Prussia. Following that conquest, Germans and Poles moved in as settlers; thus began a two-hundred-year process of Germanising this part of Eastern Europe.
Today, the order has its main presence in Vienna as a Catholic hospital order. (There is also a Protestant branch in Netherlands.1) It still retains knights to this day, who must pay for the privilege of being in the Order, and they control several schools and hospitals. In emergencies such as war, they forgo their predecessor's sword-wielding and rampaging ways and provide an ambulance service, returning to their original role as hospitallers.
The angularity of this cross makes it popular for logo design, monograms (such as ICXC), or a date for the year's Paschal Candle.