They indicate the direction of the Cartesian axes x, y, and z, respectively. In terms of these, any vector a can be represented as
where ax, ay, az are called the vector components (or vector projections) of a on the Cartesian axes x, y, and z (see figure), while ax, ay, az are the respective scalar components (or scalar projections).
A hat above the symbol of a versor is sometimes used to emphasize its status as a unit vector (e.g., ).
In most contexts it can be assumed that i, j, and k, (or and ) are versors of a 3-D Cartesian coordinate system. The notations , , , or , with or without hat, are also used, particularly in contexts where i, j, k might lead to confusion with another quantity. This is recommended, for instance, when index symbols such as i, j, k are used to identify an element of a set of variables.