CAD ("Computer Aided Design") in its most basic form is the electronic equivalent of traditional penandpaper technical drawing. CAD stores drawn shapes (such as primitive lines, points, curves) as precise mathematical representations or "vectors". Whilst this might seem an overlysimplistic description for anybody familiar with CAD, this description is as true now as it was in the late fifties when the idea was first germinated.
That a technical drawing or "mathematical representation of real world metrics" could be electronically stored, transmitted, reproduced, manipulated, merged, transformed and have calculations carried out on it with absolute mathematical precision was a profound innovation. It soon revolutionised industries such as automotive, aerospace, architecture, civil engineering, cartography, military, travel, even graphic design and desktop publishing plus countless other disciplines. Consequently, this precision lent itself perfectly to engineered instrument designs, CNC manufacture, virtual product simulation/testing and even  as a distant cousin  the PLEK system.
The tools for CAD work have become accessible to the point of ubiquity with many free CAD packages broadly compatible with high value industry standard software. Autodesk (makers of AutoCAD) even provide an online version of their software ("AutoCAD 360") offering basic sketching, modification and collaboration tools. It is no longer true to say that CAD requires the same financial, educational or time investments or that it is hindered by clunky user interfaces, is unreliable and buggy, or worst of all opaque, unhelpful with a steep learning curve. Whilst some software might seem to hang onto these negative '80s aspects of CAD, the general landscape is now completely the opposite; CAD is completely accessible and easy to pick up.
tl;dr version
CAD was a revolutionary design invention, initially for specialised industries but now a freelyavailable tool.
For the purposes of planning and drawing a personal instrument, the most basic geometry tools available in virtually all CAD packages are more than sufficient. Even if you want to go down the completist route, this too is not too far removed from the most basic tools. For example, you might want to draw out every last screwhole, inlay, radius, fret, cutaway and crosssection to reliably communicate a final design to a third party. We won't be going into this level of detail, purely because our hypothetical CAD journey is to demonstrate taking a practical design from the computer through to the wood, only highlighting details where they are genuinely useful for this end purpose.
Vectors
Without going into the deep (and unnecessary) history of Euclidean geometry or mathematics, vector geometry is simply the representation of positions and shapes on a 2D plane. Not entirely unlike dottodot on a sheet of graph paper. Unlike graph paper however, CAD coordinates/vectors are almost infinitely resolvable; numbers can be stored to staggering levels of precision. It is possible to zoom into a drawing pretty much to the limits of yours software's ability (more often, willingness) to do so without "losing resolution" as you would with a pixelbased drawing.
In real terms for woodworkers, this might seem like splitting hairs. We couldn't care less about losing 0,01mm here or 1/2048" there. Those are impossible to reliably achieve in the wood anyway; these individual measurement tolerances are genuinely not that useful in the workshop. That said, the ability at the design stage to maintain precision, prevent compound tolerance errors and to ensure that the drawings and layouts we do transfer to timber are free of significant error margins, tolerances and inaccuracies is paramount. It is perfectly reasonable to take measurements in the order of 0,25mm or 1/64" to the workshop, however being this coarse at the design stage could leave you a few mm or a large fraction of an inch out later on in the game!
To create an awfully mixed metaphor,
"looking after the pennies at the design stage means that the pounds will look after themselves in the workshop"
(groan)
Putting this together, CAD software handles all of the vector mathematics for you at huge precisions without you having to worry about having to work with big numbers or nasty maths. It's happy to locate all of your fret slots to more decimal places than you could ever use behind the scenes but leave you with two or three in your working measurements  if that's what you want in the workshop.
Model Space/Paper Space
As touched on earlier, a basic 2D CAD drawing comprises a representation of a flat XY space and a corresponding measurement system (most commonly Imperial and Metric). This environment is often referred to as the "model space" where all of your individual drawing objects are created, manipulated and navigated around in a virtual space using a realworld scaling. A model space is normally able to be divided into separable layers grouping and arranging parts of the drawing, not entirely unlike having drawings separated amongst overlaid acetate transparencies for an overhead projector.
A "paper space" is subtly different in that it contains a subsection (sometimes more than one) of your model space; a "window" or defined view into your drawing. A direct example of this may be a focus on a specific part of your model space such as the headstock. The term "paper view" originates from its major use; a printable section of the model space. A single model space can have many paper spaces, with each paper space showing different sections or selected layers from the model space plus annotations relevant to that sheet such as scaling, tolerances, filename, revision dates, etc.
The model space is where fundamental design work is done whereas the paper space is how parts of the model space are brought out into the real world as useful drawings.
The following images show an example of a finished drawing's model space with an example paper space showing a subsection of the model space and a specific selection of its layers, ready for printing.
Drawing Entities
A drawing in the model space is populated with a number of mathematicallypositioned and dimensioned primitive shapes such as points, lines, polylines, rectangles, arcs, etc. In additional to individual primitives, there are compound, complex or specific entities. A hexagon for example, is a compound of six lines in a specific geometric arrangement. A closed semicircle is a compound of a arc plus a line; in fact, a circle is usually no more than a complete arc. More specialised entities like Bezier curves and splines, etc. provide a comprehensive set of objects from which any object can be formed.
Individual CAD packages provide many different drawing tools to simplify the creation of complex shapes. Underneath this, the same primitives are at work. TurboCAD for example, can draw a rectangle with rounded corners of a specific radius. Definitely useful for drawing a soapbar pickup cavity! Underneath this convenient veneer is a set of four lines and attached 90° arcs.
The following photo shows a selection of primitives and (slightly) more complex objects:
Hopefully it now seems clearer that creating a complex instrument design requires little more than knowledge of a few basic shapes and tools. Coupled with a bit of basic geometry and maths (which you can make the software carry out...phew) plus an organised approach, CAD becomes an powerful, efficient and still a creative tool for designing your next instrument.
Designing Guitars In CAD I  CAD Fundamentals by Carl Maltby is licensed under a Creative Commons AttributionNonCommercialShareAlike 4.0 International License.
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