A birch rod (often shortened to "birch") is a bundle of leafless twigs bound together, much like a bunch of flowers, to form an implement for birching or flagellation.
In BDSM, the term birch is also used for an implement consisting of a bundle of thin canes (illustrated).
A single branch, on the other hand, when used as a disciplinary rod is known as a switch, if equally flexible, or else as a cane, cudgel or stick.
Contrary to what the name suggests, a birch rod is not necessarily made from a birch tree, as was the case with the Roman fasces, but can also be made from various other strong but flexible trees or shrubs, such as willow (hence the term willowing).
A hazel rod is very tough, and therefore particularly painful; it was used on the Isle of Man as an instrument of legal punishment until 1975, the last place in Europe to use the birch.
Another parameter for the severity of a birch rod is its size - i.e. its length, weight and number of branches. In some penal institutions, several versions were in use, which were often given names (rather like cane types). For example, in Dartmoor Prison the device used to punish male offenders above the age of 16 - weighing some 16 ounces and a full 48 inches long - was known as the senior birch.
There are several versions about the sense of soaking the birch in liquid before use, but as it takes in water the weight is certainly increased without compensatory air resistance, so the impact must be greater if the caner can use sufficient force.