A guitar neck made of wood will bend due to mainly 2 factors, atmospheric changes and the pull created by changing to a different gauge of guitar strings. A truss rod keeps the neck straight by countering the pull of the strings and natural tendencies in the wood. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921, though the idea of a "truss rod" appears in patents as early as 1908.
You always need some relief in combination with a certain action. In case you are a hard strummer you will need high action with more neck relief in order to get the maximum buzz free power out of your instrument. Relief is typically measured as the distance between the string and the 7th fret while holding down the first and 14e fret. It should be about 0,2mm (0.007”) for a standard setup.
The first picture shows a Levin 315 archtop guitar from 1959. They use an adjustable truss rod in combination with a bolt on neck construction. A unique feature, and I am still thankful they did that because it made the neck reset I did last week on this guitar much easier
Tone Wood by Frans Elferink
Frans Elferink 13 Questions