This is an excerpt from “With All the Precision Possible: Roubo on Furniture Making” by Donald C. Williams, Michele Pietryka-Pagán & Philippe Lafargue.
The elongation of wood should also be put among the number of assemblages, its application being very useful, given the impossibility of always having wood of the necessary length, or supposing that it is, the defect being that they sometimes are
not of a perfect quality along the entire length, but being corrected by this method.
There are two ways to elongate wood: the first, by notching half of each piece with tongue and grooves at the ends of each piece of wood, which you hold together by means of glue and pegs, Figs. 1 & 4.
The second way to elongate the wood is with Jupiter’s thunderbolts (apparently named thus because the shape of the cuts is a bit similar to that which you give to the gap which you wish to represent). [This is a notched and pegged scarf joint, most likely named because the configuration of the joint looks somewhat like a lightning bolt.]
There are two types of Jupiter’s thunderbolts, one which you make by notching half of each piece and by forming a second notch to receive the [inserted tapered] key. One must note to make this second notch off-set toward the end of the piece, so that the key forced against it finds no resistance in the opposite side of the other notch, and consequently it better draws the joints together [so that it acts like a draw pin], Figs. 2 & 5.
The second way is to trace in the middle of the piece two parallel lines a–b, c–d, which give you the thickness of the notch. After having determined the length of the notch, and having traced the position of the key in the middle, you cut out all the wood from the front of the wood (assuming you are looking at the front of the notch) up to the first parallel line. From the position of the key up to distance e, you make a second notch a–e, such that in each piece, what there is more of takes the place of what there is less of in the depth of the notch, and makes space for the key. For the ends of these notches, they make tongue and grooves, or only an angle, but the little tongues are better, Figs. 3,6 & 7.
This second way is very strong, and is much better than the first because the key bears all the thickness, instead of the other way, which has only half as much. What’s more, a key bearing only half [the thickness] is subject to rolling, and consequently to open the joint. Even if the joint does not open up, the key can be eaten up [word down] and forced, bearing on the opposite side of the groove, which loses its desired effect, see the figures above.
This assembly is very useful and very strong, and is in use not only by Joiners, but also by carpenters, as much for buildings as for ships.
When the entire length of the wood which you wish to elongate is taken up by mouldings, and you cannot or do not wish to make Jupiter’s thunderbolts, for fear that the key and the grooves will not meet up in the mouldings, you use an assembly called a flute, or a scarf joint, which is made in this way.
After having divided the width of your piece into two equal parts, as indicated by line f–f–g, you make the length that you wish to give to your grooves by h–i–l–m. From this line to the end of your piece, you draw diagonals r–o–p–i, and f–q–m–n, some from one side of the line and the others from the other, such that these notches are made in two pieces with much precision, are at the same time a solid and very tight assembly. You must take care that these grooves be made going from right to left, so that when you wish to elaborate with mouldings, they will not be subject to splitting, Fig. 8.
Although I said that you must separate the piece into two pieces to make these types of notches, this rule is not however general.When you have many pieces of mouldings in the piece, you put the joint in the loosening of one from the other, if it is found in the middle, or in the middle of the groove, as you can see in Fig. 9.
When you elongate pieces ornamented with mouldings using Jupiter’s thunderbolts, you should take care to make notches according to the depth of the moulding, if there is not a groove, so that the key is not uncovered, Fig. 10.
You can also lengthen curved pieces, both on their face and on their edge, using Jupiter’s thunderbolts, as indicated in Figs. 11 & 12. For as many pieces as are curved on the face, and for as little as they are curved, you should never make any tenons, because they will become too sliced up, and consequently less solid. You should fit them together by making at the end of the piece a forking of little depth and of the thickness of the tenon. In this forking you make three or four holes for placing pegs or dowels from the tenon that you fit together. These types of tenons are called tenons a peignes [toothed tenons, doweled tenons], Fig. 12.
There you have it, all the different assemblies that are used for the construction of joinery. I have detailed them the best that was possible for me; this matter, lifeless by itself, not being able to be rendered with as much clarity as I would have wished. You will have recourse to the plates where I have illustrated all the different assemblies, either joined or separated, so that you can see their effect better. I have also indicated all those that are hidden by punctuated lines. I hope that for as little as you may wish to pay attention, the demonstration that I have made will supplement that which one could find obscure in this discussion.
— Meghan Bates