The trussrod is one of those components that if all goes well the player doesn't even notice them, but if a problem arises and you need it it can make or break a guitar if it does not perform.

It's main goal is to allow for the relief action and to counter act neck bows, and there are mainly 4 types of truss rods dived into two categories, the single rods and double rods. Each category having two typologies, the single action and double action.

 Single rod dual action by Bruce Johnson

These are more rare to find as they are more labor intensive to make and install. The single action rod has a anchor point for the rod on one side and in the other the traditional nut and washer. The double action differs from the first by having a nut that is captive in a anchor point.

Dual rod single action

This design is also very unusual, and if used by Rickenbacker. Its consists of two single rods single action side by side. Also used due to tradition of the brand.

This is the most common trussrod in use today, and this is because it allows corrections of the neck bow in both directions, but also it's very easy to install only requiring a straight slot.

Personally I favor the single rod dual action design and this is because it allows corrections in both directions, its lighter then the dual rod and because it does not have the potential damping effect on the vibrations that the dual rod designs can have.

Let's take a look at some of the early patents related to truss rod designs.

1896 T Moore

This is where this look at the trussrod development starts, on what the authors called “Neck Brace”. This is the oldest patent drawing about trussrod's that I found in my research so far, and it is an exterior approach to the design, not yet inside the neck.

1897 Benson

The second oldest patent that I found relating to the truss rod is a non-adjustable metal profile. This would help make the neck more resistant to twist and bows, but not adjustable. May I call your attention to the detail where the strings pass through the bridge! On this design you would load the string from the tail/bottom of the body, inside the box.

1898 Bremerman

Bremerman proposed a different approach in 1898. He proposed to use a laminated neck, which was nothing new, but with a stronger material in the centre, he mentioned ivory, ebony or aluminium. The peculiar part is that he said the part should extend outside the neck, resulting in a neck centrepiece thicker than the neck itself, and I doubt any player in the world would find that comfortable…

1921 MC Hugh

Now this is the famous M.C. Huch patent for Kalamazoo, and is in fact the first approach to elegantly provide a solution to neck adjustment. This is the base of what we use today, so I guess it was spot on!

1937 Stathofoulo

 Another version soon come out, this time by Stathofoulo, and is a trussrod adjustable from inside the body, with the detail that the acting part of the truss rod occurs only on the neck after the heel.

1959 Jim burns

This is actually the principles of the single rod dual action design.

1962 Jim burns

A different version also by Jim Burns, that allows for dual action. Here he added a worm gear to allow the rod to be adjusted from the bottom of the neck heel. There are version of these today to adjust it from the side. 

1962 Hagstrom

 Hagstrom take on the trussrod.

 The famous Fender trussrod design! It's a single rod, single action installed in a curved slot. Why they still use it is the question. They do have a version of it called Bi-flex that is similar to the double action rods but is present in very few guitars.

Fender Bi-Flex design

1966 Baldoni 

 In this Baldoni patent we see the typology of dual rod single action style of truss rod.