While there is great variety in the construction details of bows (both historic and modern) all bows consist of a string attached to elastic limbs that store mechanical energy imparted by the user drawing the string. Bows may be broadly split into two categories: those drawn by pulling the string directly and those that use a mechanism to pull the string.
Directly drawn bows may be further divided based upon differences in the method of limb construction, notable examples being self bows, laminated bows and composite bows. Bows can also be classified by the bow shape of the limbs when unstrung; in contrast to simple straight bows, a recurve bow has tips that curve away from the archer when the bow is unstrung. The cross-section of the limb also varies; the classic longbow is a tall bow with narrow limbs that are D-shaped in cross section, and the flatbow has flat wide limbs that are approximately rectangular in cross-section.
The classic D-shape comes from the use of the wood of the yew tree. The sap-wood is best suited to the tension on the back of the bow, and the heart-wood to the compression on the belly. Hence, a limb sector of yew wood shows the narrow, light-coloured sap-wood on the ‘straight’ part of the D, and the red/orange heartwood forms the curved part of the D, to balance the mechanical tension/compression stress. Cable-backed bows use cords as the back of the bow; the draw weight of the bow can be adjusted by changing the tension of the cable. They were widespread among Inuit who lacked easy access to good bow wood. One variety of cable-backed bow is the Penobscot bow or Wabenaki bow, invented by Frank Loring (Chief Big Thunder) about 1900. It consists of a small bow attached by cables on the back of a larger main bow.
A compound bow is a bow designed to reduce the force required to hold the string at full draw, hence allowing the archer more time to aim with less muscular stress. Most compound designs use cams or elliptical wheels on the ends of the limbs to achieve this. A typical let-off is anywhere from 65%–80%. For example, a 60-pound bow with 80% let-off will only require 12 pounds of force to hold at full draw. Up to 99% let-off is possible.
The compound bow has become the most widely used type of bow for all forms of hunting in North America. The compound bow has become a highly popular form of archery, so much so that it is the most commonly used bow form in archery today. The compound bow was first developed in 1966 by Holless Wilbur Allen in Missouri, and a US patent was granted in 1969.
Mechanically drawn bows typically have a stock or other mounting, such as the crossbow. They are not limited by the strength of a single archer and larger varieties have been used as siege engines.