= Wingspan
{title2=$b$}
The \b[wingspan] $b$ of an <aircraft wing> is its lateral tip-to-tip extent. For a symmetric <wing planform>, a spanwise coordinate $y$ runs from $-b/2$ at one tip to $b/2$ at the other; the semispan is $b/2$. This geometrical distance differs from the local fore-and-aft <airfoil chord>.<Wing Geometry by NASA Glenn Research Center>[Top View: tip-to-tip span and chord directions]{ref}
Together with <wing area> $S$, the <wingspan> defines the dimensionless <wing aspect ratio>,
$$
AR=\frac{b^2}{S}.
$$
For a rectangular <wing planform> of constant <airfoil chord> $c$, $S=bc$, giving $AR=b/c$. For example, a wing with $b=12\,\mathrm{m}$ and $S=18\,\mathrm{m^2}$ has $AR=8$. A second wing with the same <wing area> but $b=9\,\mathrm{m}$ has $AR=4.5$. Equal <wing area> therefore does not imply equal lateral slenderness.
There is also a simple structural reason why the span matters. Suppose a symmetric wing supplies total upward <lift> $L$, distributed uniformly along its span. On one half-wing the resultant load is $L/2$ and acts $b/4$ from the center. The corresponding bending moment at the center is
$$
M=\frac L2\frac b4=\frac{Lb}{8}.
$$
For $L=4000\,\mathrm{N}$ and $b=12\,\mathrm{m}$ this gives $6000\,\mathrm{N\,m}$. This is an idealized aerodynamic loading calculation: the actual load distribution, structural supports and component masses modify the wing's bending moment. It nevertheless shows why increasing <wingspan> affects the structure as well as the <wing aspect ratio>.
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