Q: What is "Safe Triangle" to insert chest tube?

Answer: British Thoracic Society recommends the tube is inserted in an area described as the "safe triangle", a horizontal line at the level of the nipple and two muscles of the chest wall i.e., latissimus dorsi and pectoralis major. 

Reference:

Laws, D. "BTS guidelines for the insertion of a chest drain". Thorax 58 (90002): 53ii–59