背筋

1. Origins, structure, and basic meaning

• Structure: 背筋 is read as せすじ and is a noun for the line of the back or spine, also used in expressions about posture and a chill running down the spine.
• Basic usage: Straightening the back appears in 背筋を伸ばす, while feeling a chill down the spine appears in 背筋が寒くなる.

2. Common pitfalls and similar expressions

• Posture and an idiom of fear
  • Beyond the body part, it appears in expressions for standing straight and for the bodily chill caused by fear.
• Difference from 背中
  • Compared with the broad surface of the back, this term emphasizes the central line running along the spine.