The butt joint is the simplest form of joinery. When the end of a square-cut (90 degrees) piece of lumber (#1) needs to be fastened to the face of another flat board (#2), the (#1) piece simply “butts” into the face of the other board (#2). Since a butt joint has no real integrity by itself, hardware fasteners must be used sometimes wood glue or construction adhesive is needed, as well.
They are used extensively in house framing. Squared end cuts of dimensional framing lumber — like 2 by 4’s, and 2 by 6’s for wall framing, and 2 by 8’s and 2 by 10’s for building floor joist framework — are butted into outside board faces, and fastened with nails or screws.
When doing wall framing work, the carpenter will nail the butt joints together using a method called “toe-nailing”, which simply means nailing through the end of a board at a 45 degree angle, tying the two boards together like a “T”. Floor joist butt joints usually require the use of “joist hangers”, which are zinc plated or galvanized steel brackets, that work like a saddle.
It should be noted that boards joined together with these joints aren’t always square cut.
Roof trusses are a good example and trusses generally call for different fasteners. Roof truss boards are usually joined together with steel plates that have tiny spikes or “prongs” on them. The steel plates are stamped this way at the factory.
These steel plates are simply hammered over the whole intersection of the joint, tying the two boards together, and sometimes supplemented with nails or screws.
Smaller versions of these steel plates, used to connect butt joints together on smaller wood projects, are called “mending plates”.
Mending plates typically have counter sunk holes in them, for using screws, the mending plate is placed over the butt joint, and screwed into place.