~Tension~ results when a pulling force is applied to opposite
ends of a body. This external pull is communicated to the
interior, so that any portion of the material exerts a pull or
tensile force upon the remainder, the ability to do so depending
upon the property of cohesion. The result is an elongation or
stretching of the material in the direction of the applied
force. The action is the opposite of compression.

Wood exhibits its greatest strength in tension parallel to the
grain, and it is very uncommon in practice for a specimen to be
pulled in two lengthwise. This is due to the difficulty of
making the end fastenings secure enough for the full tensile
strength to be brought into play before the fastenings shear off
longitudinally. This is not the case with metals, and as a
result they are used in almost all places where tensile strength
is particularly needed, even though the remainder of the
structure, such as sills, beams, joists, posts, and flooring,
may be of wood. Thus in a wooden truss bridge the tension
members are steel rods.

The tensile strength of wood parallel to the grain depends upon
the strength of the fibres and is affected not only by the
nature and dimensions of the wood elements but also by their
arrangement. It is greatest in straight-grained specimens with
thick-walled fibres. Cross grain of any kind materially reduces
the tensile strength of wood, since the tensile strength at
right angles to the grain is only a small fraction of that
parallel to the grain.