walk the line

verb4

1. •
   To mark or secure a boundary by walking along it.

   • And on a day we meet to walk the line And set the wall between us once again. — 1914
2. •
   To act in a non-destructive or damaging manner, whether physically or mentally to those you hold dear and/or respect; to exercise self-control.

   • I keep a close watch on this heart of mine I keep my eyes wide open all the time I keep the ends out for the tie that binds Because you're mine, I walk the line. — 1956
3. •
   To maintain an intermediate position between contrasting choices, opinions, etc.

   • I sat at dinner, but satisfied myself with nibbling bread crusts, and witnessing the forlorn and perilous efforts of my friends to walk the line between starvation and acute indigestion. — 1914
4. •
   (US) To participate in the procession at a graduation ceremony; to graduate.