A split-rail fence, log fence, or buck-and-rail fence (also historically known as a zigzag, worm, snake or snake-rail fence due to its meandering layout) is a type of fence constructed in the United States and Canada, and is made out of timber logs, usually split lengthwise into rails and typically used for agricultural or decorative fencing. Such fences require much more timber than other types of fences, and so are generally only common in areas where wood is abundant. However, they are very simple in their construction, and can be assembled with few tools even on hard or rocky ground. They also can be built without using any nails or other hardware; such hardware was often scarce in frontier areas. They are particularly popular in very rocky areas where post hole digging is almost impossible. They can even be partially or wholly disassembled if the fence needs to be moved or the wood becomes more useful for other purposes.

Split rail fences were made of easy to split, rot-resistant wood. Traditionally American chestnut was the timber of choice until chestnut blight eliminated this tree. Currently, most split rails are made from cedar. Whether of chestnut or cedar, these logs were cut to a length of 10 to 12 feet (3.0 to 3.7 m) and split down the length of the log. Each half was then split into quarters, then eighths and so on until the rails were of a usable size. A log may produce from four rails from an 8-inch (20 cm) log to over a dozen from larger logs. The rails are stacked on top of one another. Most split rail fences have the rails stacked in an interlocking zig-zag fashion that is self-supporting, easy to create, easy to repair, and easy to disassemble.

Some timber fences have the rails stacked directly on top of each other and secured with double fence posts (one on either side of the rails). This made a more permanent and compact fence but remained easy to repair.

The distance between either the zigs or the zags is generally 16.5 feet (5 m) or one rod. The area of a field can therefore be calculated by counting zigs or zags along the side and end of the field: one hundred sixty square rods is 1-acre (0.40 ha).