# Standard basis

These vectors are a basis in the sense that any other vector can be expressed uniquely as a linear combination of these. For example, every vector **v** in three-dimensional space can be written uniquely as

where **e**_{i} denotes the vector with a 1 in the ith coordinate and 0's elsewhere.

By definition, the standard basis is a sequence of orthogonal unit vectors. In other words, it is an ordered and orthonormal basis.

However, an ordered orthonormal basis is not necessarily a standard basis. For instance the two vectors representing a 30° rotation of the 2D standard basis described above, i.e.

are also orthogonal unit vectors, but they are not aligned with the axes of the Cartesian coordinate system, so the basis with these vectors does not meet the definition of standard basis.

There is a *standard* basis also for the ring of polynomials in *n* indeterminates over a field, namely the monomials.

from *I* into a ring *R*, which are zero except for a finite number of indices, if we interpret 1 as 1_{R}, the unit in *R*.

The existence of other 'standard' bases has become a topic of interest in algebraic geometry, beginning with work of Hodge from 1943 on Grassmannians. It is now a part of representation theory called *standard monomial theory*. The idea of standard basis in the universal enveloping algebra of a Lie algebra is established by the Poincaré–Birkhoff–Witt theorem.

In physics, the standard basis vectors for a given Euclidean space are sometimes referred to as the versors of the axes of the corresponding Cartesian coordinate system.