The grading of wood floor timbers can sometimes be rather confusing, with different manufacturers often having different terms definitions for their grades of timber. Also, grading may also differ between different species of wood where the grain, knot or colour characteristics of any given timber don't have an equivalent in another species.
Notwithstanding this, as a general guide, wood floors can broadly speaking be arranged into 3 grades;
Prime or Classic Grade:
This is the highest grade and will generally be clean and straight-grained with no sapwood. Even tone and colouration. No knots, unless these are very specific to the aesthetics of the species and are deemed as desirable. Also, please note that some timbers with wide natural colour variation may be separated and sorted into several classic grades each with a different colour.
Select or Nature Grade:
Some grain variation, some colour variation, with up to 10% sapwood allowed in some species. Only small knots (generally less than 10mm diameter) allowed. Generally a relatively clean finish, and sometimes easily mistaken for Prime grade. Often preferred over Prime for the subtle grain characteristics.
Rustic or Character Grade:
Distinguished by an increase in the number and variety of size of knots (some up to around 30mm diameter). Other features may include unlimited colour variation, including sapwood and dark streaks. Large and cracked knots are most often filled with resin but are structurally sound. The most popular grade for its lively and authentic appearance.
Extra or Super Rustic Grade:
Similar to Rustic grade, but distinguished by the inclusion of cracked knots, splits and shakes. Only available as an unfinished board, cracked knots and splits are unfilled but if particularly severe can be selectively filled at point of installation. Despite its gnarly appearance, a very attractive grade popular in barn conversions and cottages.