By contrast, it is always possible to pull back a differential form. A differential form on may be viewed as a linear functional on each tangent space. Precomposing this functional with the differential defines a linear functional on each tangent space of and therefore a differential form on . The existence of pullbacks is one of the key features of the theory of differential forms. It leads to the existence of pullback maps in other situations, such as pullback homomorphisms in de Rham cohomology.