A well secured boat will have up to eight dock lines: two breast lines at the bow, two breast lines at the stern, and two spring lines on each side. The spring lines run from a boat cleat amidship to dock cleats or pilings near the bow and stern. If there is a pier (or there are pilings) on only one side, four lines can be used: one from the bow, one from the stern and two spring lines; however, four is the bare minimum.
When the boat is tied up on only one side, fenders are used to hold it off the dock or pilings. They should be dangling vertically at a dock or rigged horizontally at pilings.
Breast lines at the bow and stern prevent motion from side to side. They deal mainly with wind. Spring lines prevent moving forward and backward. They deal mainly with tide and current. Under many conditions the paired spring lines do most of the work.
Dock lines should be nylon, because stretch is wanted to relieve tugging strains, and it is usually single braided. For boats up to 30 ft, use ½-in line. From 30 to 40 ft, use 5/8-in line. From 40 to 70 ft, use ¾-in line. Avoid oversize line that might not stretch enough.
Breast lines are usually about 2/3 of the boat length and spring lines are the full length of the boat. Dock lines tend to chafe through quickly and need some sort of chafe protection. A length of stout hose or tubing, larger than the line and drilled for twine at both ends, works as well as anything. Keep it in place by tying the twine securely to the dock line.