The grain you see on any piece of wood is actually the growth rings of the tree. These are created by the cycle of growth and dormancy a tree experiences with the change of seasons. Grain is the unique identifying marker of each and every tree on Earth. Every tree has differing growth experiences and therefore differing grain characteristics.
Various milling techniques are used by timber mills to extract the lumber from a tree. The techniques may focus on efficient use of the raw materials, or highlighting the natural beauty of the wood.
Rift and quartered milling uses milling techniques to highlight the most beautiful grain patterning possible. Typically, rift and quartered is known to be a very traditional milling process. Flooring milled in this way is common in older homes and is what generally comes to mind when hearing "hardwood flooring". However, it is achieved through one of the least common milling styles and is actually not as common as plain sawn flooring today. For more information on milling types.
Rift and Quartered grain is achieved through a specific milling technique that differs from the more common, plain sawn style. Logs are cut with the grains intersecting the face of the board at an approximate 60-degree angle. The log is sawn into four quarters and each quarter of the log is sawn perpendicular to the growth rings at an angle, therefore being named rift and quartered. This creates the traditional long lined grain that rift and quartered is known for.
Another distinct attribute of rift and quartered hardwood is the presentation of medullary rays. Medullary rays are the cellular structures found in woods, consisting of small capillaries inside wood that run from the center of the tree to the outer growth ring. When quarter sawn, they create a wavy, ribbon-like effect in the board known as "flecking". Hardwood only has a straight grain with flecking when sawn into four quarters and cut at a 60-degree angle. In other words, only rift and quartered grain can have a straight grain with flecking because of how it's milled.
Many wood species can be cut in a rift and quartered grain; it is often known to be more stable with less expansion and contraction cause by changes in temperature and humidity. Making it more desirable and usable in a wider range of applications. While rift and quartered grain is most common with White Oak, Tesoro Woods' Great Northern Woods Collection features four North American species in rift and quartered grain in both 3" and 5" widths.