It will not take you very long to realize that most timbers tend to work better 

A cut is always more effective when it is directed from short grain to long.

 in one direction than the other. Timber such as oak for example, may plane beautifully one way leaving a smooth surface and a polished sheen from the blade. However, if you try to plane in the opposite direction, the grain may lift, producing sporadic tears and pitted areas. The same problem will also occur when carving a piece of wood with a chisel, making the process frustrating as well as leaving you with a difficult task in terms of repairing the damaged surface of the wood. Although one should always aim to carve in the best direction, on certain projects such as mouldings, it is often impossible. This is where the sliding technique comes into play. 

To carve timber in the most effective directions you must first understand how the timber is formed. This, in turn, will help you make decisions about the appropriate direction to cut. 

Imagine a piece of wood as being a bundle of long drinking straws, the length of the straws representing the direction of the grain. Figs. 1-3 show that the cut will always be more effective when it is directed from short grain (straws) to long. As you can see in Fig. 2, working in the correct direction produces a smooth, clean cut while working from long grain to short as shown in Fig. 3 will result in a ragged cut.

Carving long grain to short
Carving short grain to long