The fourteen central frames of the San Juan are placed on top of the middle of the keel. Each one is made up from a floor timber and its corresponding two ribs, also called first futtocks, on each side. They are joined by a dovetail scarph plus two 12" wrought-iron nails and two wooden pegs. The one in the middle is more than 7 meters wide and weighs 800 kg; it's called the main frame.
The main frame
We could say that the main frame is the most important of all. It is directly linked to the ship's maximum width, also called the beam. Instead of having one futtock attached on either end of the floor timber, the main frame has two.
The ship's beam was the most important measurement of the whole ship, and its size determined all the other parts of the ship. At the time, when no plans or other graphic representations were used in shipbuilding, the master shipbuilder had absolutely all the proportions of the ship in his head: the length of the keel was double the beam, the length of the hull three times that of the beam, the height of the upper deck three quarters of it and so on. Every master made their personal adjustments to these proportions, but the beam was always the primary reference. That's why tracing and making the main frame is so important.
All frames are numbered starting from the main one, which is zero. The six ones towards the ship's bow are numbered from one to six, while the seven ones towards the rear are numbered from minus one to minus seven.
Installation of the central frames
The frames are placed on top of the keel at a distance of approximately 8". Their correct positioning and aligning is essential. Later they'll be nailed and pegged to the keel from the outside.
More pictures of building process here