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Picanha is a cut of beef popular in Brazil. In the U.S. it is little known, but referred to as the rump cover or rump cap. North American butchers generally divide it into other cuts like the rump, the round and the loin. However, due to different cattle breeds produced in the USA, in some places it is named as "top sirloin cap". 
In Brazil, the most prized cut of meat tends to be the picanha, which is the rump cap. It is, in fact, the capping muscle over to the sirloin/rump area. It is the topmost layers of muscle covered in a layer of thick fat.
In Brazil, the fat is not removed from the cut until the steak has been cooked. In the U.S., however, the fat tends to be cut down at the butcher shop unless otherwise directed by the customer.
To locate the picanha cut more specifically, one must realize that the capping muscle actually has a right side and a left side. One side (the larger side) is called the tri tip, and the other side (the smaller side) is the Brazilian picanha.
In the U.S., the picanha is often called the top sirloin cap (culotte or coutlotte) but the most accurate translation is the rump cap. It is the smaller rump cap that is on the side opposite the tri tip cut. Thus, one can see that the tri tip and the picanha cuts are in fact different (albeit similarly located) cuts of beef.
The Picanha should not be confused with Top Sirloin. The Picanha comes from the "cap" (or "culotte"), which lies above the top sirloin and rump areas. This is actually a different cut of beef and can be easily distinguished by the thick layer of fat on one side (typically many centimeters thick). Similarly, and as previously mentioned, the tri tip is a different cut. However, many consider the tri tip to be the best substitute for the picanha due to the fact that it is also a cap muscle that is located on a similar part of the cow's body.