@B00609@ in the system of quantities upon which SI is based. Special symbols are used for different types of lengths.
Sources: Green Book, 2nd ed., p. 11 [Terms] [Book] PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 978 [Terms] [Paper]