A basic technique for designing curved shapes in the plane is
\emph{interpolating splines.} The designer inputs a sequence of
control points, and the computer fits a smooth curve that goes through
these points. The literature of interpolating splines is rich, much of
it based on the mathematical idealization of a thin elastic strip
constrained to pass through the points. Until now there is little
consensus on which, if any, of these splines is ideal. This thesis
explores the properties of an ideal interpolating spline. The most
important property is \emph{fairness,} a property often in tension
with \emph{locality,} meaning that perturbations to the input points
do not affect sections of the curve at a distance. The idealized
elastic strip has two serious problems. A sequence of co-circular
input points results in a curve deviating from a circular arc. For
some other inputs, no solution (with finite extent) exists at all.

The idealized elastic strip has two properties worth
preserving. First, any ideal spline must be \emph{extensional,} meaning
that the insertion of a new point on the curve shouldn't change its
shape. Second, curve segments between any two adjacent control points
are drawn from a two-parameter family (and this propety is closely
related to good locality properties). A central result of this thesis
is that any spline sharing these properties also has the property that
all segments between two control points are cut from a single, fixed
generating curve. Thus, the problem of choosing an ideal spline is
reduced to that of choosing the ideal generating curve. The Euler
spiral has excellent all-around properties, and, for some
applications, a log-aesthetic curve may be even better.

Shapes in applications such as font outlines contain extra features
such as corners and transitions between straight lines and smooth
curves. Attaching additional constraints to control points expresses
these features, and, carefully applied, give the designer a richer
palette of curve types.

The splines presented in this thesis are entirely practical as well,
especially for designing fonts. Sophisticated new numerical techniques
compute the splines at interactive speeds, as well as convert to
optimized cubic B\'ezier representation.