January 2013
Scalar quantities are usually set in lightface type.
Three-vectors are generally set in boldface type; alternatively, an arrow over the character may be used ($\mathop{{\rm{x}}}\limits^\rightarrow$). Unit vectors are also generally set in boldface and are distinguished from ordinary three-vectors by a caret (the preferred notation is lowercase boldface e, i.e., $\hat{e}$). A unit vector in the direction of an already defined vector k is thus denoted by $\hat{e}_k$ or by the same letter with a caret, i.e., $\hat{k} = k/|k|$. Four-vectors and the magnitudes of components of all vectors are set in lightface type.
More general vectors, matrices, etc., are usually set in lightface; boldface may alternatively be used to distinguish one from the other. If neither the context nor the typeface makes the distinction sufficiently clear, a distinguishing mark may be used.
Second-rank tensors are usually set as capital sans serif letters (A).
The sets of complex, real, etc., numbers may be denoted by the standard open-font symbols $\mathbb{C, R}$, etc.
Group representations signified by numerals (e.g., 8 for the eight-dimensional representation) are set in boldface.