A quark index is relevant because it is often more convenient to use a number instead of a letter to represent different kinds of quarks. So consider a seed noted by Z where
Ⓐ, Ⓑ, Ⓘ, Ⓦ, Ⓓ, Ⓛ }
The seed Z can be used to define an ordinary quark written as and its associated anti-quark . These quarks are occasionally referred to using , the Greek letter zeta, as shown in the table below. When used like this, zeta is called a quark index. The two particles and are sometimes collectively called -type or Z-type quarks. This notation is especially helpful when using summation notation in formulae.