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spine; line of the backbone
seam down the back (of clothing)
「背筋」 has another reading with different meanings.
muscles of the back; back muscles
背筋せすじを伸ばしのばして座りすわりなさい。
Sit up straight.
恐ろしくおそろしくて背筋せすじがぞっとした。
It was so scary that a chill ran down my spine.