We study the spin-$1/2$ Ising chain with multispin interactions $K$ involving the product of $m$ successive spins, for general values of $m$. Using a change of spin variables the zero-field partition function of a finite chain is obtained for free and periodic boundary conditions (BC) and we calculate the two-spin correlation function. When placed in an external field $H$ the system is shown to be self-dual. Using another change of spin variables the one-dimensional (1D) Ising model with multispin interactions in a field is mapped onto a zero-field rectangular Ising model with first-neighbour interactions $K$ and $H$. The 2D system, with size $mtimes N/m$, has the topology of a cylinder with helical BC. In the thermodynamic limit $N/mtoinfty$, $mtoinfty$, a 2D critical singularity develops on the self-duality line, $sinh 2Ksinh 2H=1$.
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