History

The history of the triangle begins with the Egyptian Sistrum, which was possibly the ancestor of the triangle. Then, in the 10th century, the triangle was accounted for in a manuscript, which called it "An instrument without rings". Strangely, early triangles did have rings around them, to add to the sound, and the rings supposedly only disappeared in the 19th century, according to another source. They also were made of solid iron. Then, skipping far through history, it was depicted in 16th century paintings as being used in religious ceremonies alongside other instruments, and being played by angels. Finally, the triangle was used in the 1800s by composers you can find on this website/Spotify playlist below.

Current Use

Firstly, yes, there are professional triangle players, and yes, it takes skill to use a triangle. If you still aren't convinced,  you can see a clip of a professional triangle player playing their instrument here. Apparently, it is difficult to play the triangle because you must carefully manipulate the instrument to get the right timbre (the character of the sound) and articulation (how the sound is played, whether lightly or sharply) you need., although I can't confirm any thing as I am not a percussion player. Triangles are also found in folk, rock, and pop songs. They are made of steel, and are around 6-9 inches in diameter. 

"Fun" Facts

• Liszt's Piano Concerto #1 was the first classical piece to include a (subtle) triangle solo.
• The triangle has been used in some Turkish music.
• In Cajun style music, the triangle can be used instead of the drum to keep time.
• The triangle has been depicted (possibly wrongly) as being used to call others to the dinner table in the 1800s.
• Triangles are classified as idiophonic instruments, meaning, according to Merriam-Webster, "Any of a class of musical instruments (such as a bell or gong) whose sound is generated by striking, rubbing, plucking, or blowing the material of the instrument itself not under any special tension".