Wood is anisotropic: this means that it is directionally dependent and, in particular, that along different dimensions it has different material properties. One of the consequences of the fact that wood is anisotropic is that it will split more easily along the grain than against the grain. This is an important thing to understand when it comes to the science of joining two bits of wood together so that the joint is not too flimsy or easily broken. Bits of wood can be joined together in a variety of ways. Some of these ways may involve glue, screws, nails, dowels or other additional materials that help to make the joint more solid.
A butt joint is an easy joint that can be used to join two pieces of wood by butting them up against each other and gluing them. For instance, if you want to glue together two bits of wood at a perpendicular angle then make sure the end of one bit of wood is cut flat at a 90-degree angle to the face of the wood, then apply glue to the areas on each piece of wood that are going to be in contact with each other (having previously measured these areas out and marked them up carefully) then press the two pieces of wood together and keep them in good contact until the glue is dry. This is a good joint to use if you want your joint to be easy to break.
A biscuit joint is a variant on a butt joint. It doesn’t require an actual biscuit, just a biscuit-shaped bit of wood which is partially inserted into a half-biscuit-shaped hole in each of the bits of wood that are butting up against each other to reinforce the joint. For instance, if you want to glue together two bits of wood at a perpendicular angle but don’t want the joint to be quite as weak as a butt joint, then make sure the end of one bit of wood is cut flat at a 90-degree angle to the face of the wood. Then cut a circular or oval ‘biscuit-shaped’ piece of wood, thinner than the thinnest of the two pieces of wood, but thick enough to reinforce the joint. Then you need to carefully measure up and cut a semi-biscuit-shaped hole in each of the two bits of wood you are joining together, ensuring that these align in such a way that the biscuit will fit into the holes when the two bits of wood are put together in the correct position. Check that this works without glue and start from the beginning of the process all over again if it has gone wrong. If you have a biscuit and two semi-biscuit-shaped holes, one in each of the pieces of wood you are joining together, and the holes align in the correct position, then apply glue to the areas on each piece of wood that are going to be in contact with each other (having previously measured these areas out and marked them up carefully) as well as to the biscuit, then press the two pieces of wood together (with the biscuit slotted into the semi-biscuit-shaped holes between the two) and keep them in good contact until the glue is dry. This is a good joint to use if you want to undertake a time-consuming additional task that makes your joint marginally less easy to break than a simple butt joint.
A mitre joint is a kind of butt joint, but instead of gluing the two bits of wood together at a perpendicular angle, you cut the end of each of the two bits of wood to a 45-degree angle (or to two angles that add up to 90 degrees). You carefully measure up the two pieces of wood so that the two faces of the two bits of wood fit together, then carefully apply glue to each face then press the two pieces of wood together and keep them in good contact until the glue is dry. A mitre joint isn’t really much stronger than a butt joint, but it does take considerably more time to make.
A nailed butt joint is a slightly more efficient way of making a butt joint. As well as your two bits of wood and some glue you will need a big hammer and a large quantity of nails. For instance, if you want to glue and nail together two bits of wood at a perpendicular angle then make sure the end of one bit of wood is cut flat at a 90-degree angle to the face of the wood, then apply glue to the areas on each piece of wood that are going to be in contact with each other (having previously measured these areas out and marked them up carefully).