Sheets: The main, jib, and spinnaker sheets are truly the big lines, and the origin of the term. When I coach, the single biggest improvement a team can make is sail trim, even with the most seasoned teams. On our boat, my trimmer and I are incessantly monitoring and adjusting the sheets. When underpowered, we spend considerable time looking at the leech telltales of both the main and jib. When overpowered, we are adjusting them to balance the helm.