  Knitting is not only a mere art and craft hobby but also a thousand year old
technology. Unlike weaving, it can produce loose yet extremely stretchable
fabrics with almost vanishing rigidity, a desirable property exhibited by
hardly any bulk material. It also enables the engineering of arbitrarily shaped
two and three-dimensional objects with tunable mechanical response. In contrast
with the extensive body of related empirical knowledge and despite a growing
industrial interest, the physical ingredients underlying these intriguing
mechanical properties remain poorly understood. To make some progress in this
direction, we study a model tricot made of a single elastic thread knitted into
the common pattern called \textit{stockinette}. On the one hand, we
experimentally investigate its tensile response and measure local displacements
of the stitches during deformation. On the other hand, we derive a
first-principle mechanical model for the displacement field based on the yarn
bending energy, the conservation of its total length and the topological
constraints on the constitutive stitches. Our model solves both the shape and
mechanical response of the knit and agrees quantitatively with our
measurements. This study thus provides a fundamental framework for the
understanding of knitted fabrics, paving the way to thread-based smart
materials.
