According to the classic analysis of Ferdinand de Saussure, the signs that make up any given semiotic system can be broken down into two elements: the signifier (the actual sign, e.g. the word ‘dog’), and the signified (the concept that the sign represents, e.g. the dictionary definition of the word ‘dog’). A third possible element is the referent (the thing to which the concept refers, e.g. an actual dog), but for Saussure, language actually makes more sense if you think of it purely in terms of the relationship between signifier and signified, and exclude consideration of the referent altogether. What makes heraldry ‘uniquely pure’ is that it anticipates this conclusion. Nothing in heraldry claims to represent anything external to the code itself, or at least the referential aspect of the system is attenuated, as if heraldry has already evolved beyond this primitive function, which remains only in a vestigial form, like the tail on a human skeleton.
How, then, does heraldry work? It is best understood as a code, like a computer code, which is used to generate shields or coats-of-arms. The sine qua non of heraldry is therefore the shield itself, but the most basic elements of the code, which are combined to create the shield, are the heraldic tinctures, which are split into two basic groups: colours and metals. The colours are blue (azure), red (gules), black (sable), and green (vert), although the last is used far less often than the first three. Some versions also add purple (purpure) to the colours, but it almost never occurs in actual historical shields. The metals are silver (argent) and gold (or), which may also be represented by white and yellow respectively.
Every shield has one of these tinctures as its base or field, onto which are laid successive layers of additional elements – almost like layers in Photoshop – all of which are also assigned a tincture, and which cumulatively make up the shield. Thus the shield is always viewed as a single, combined image, but its component elements can always be broken down into a series of two-dimensional layers laid on top of one another in a predetermined order. From top to bottom, this order runs thus: field, ordinaries, subordinaries, charges.
Ordinaries consist of geometric devices such as a bend (a diagonal stripe across the shield), a pale (a vertical stripe down the middle of the shield), a fess (a horizontal strip across the middle of the shield), a cross, a saltire (a diagonal cross), a chevron, and so on. These are all laid on top of the field, and one of their functions is to subdivide it, so that their proportions in relation to the total area of the shield are therefore strictly controlled. For example, a pale or a fess should occupy roughly one-third of the shield’s area, while a chevron should occupy roughly one-fifth.
Subordinaries are also geometric devices. They include the chief (a horizontal strip across the top third of the shield), the canton (a small square in the dexter chief, i.e. the top left from the viewer’s perspective ) and the bordure (a thin border around the shield’s outer edge).
The final layer is that of pictorial charges. The most common of these are lions and eagles. Charges do have a notional referent, but they are always rendered in a highly abstract manner, and their relation to actual lions and eagles, or even to the symbolic meanings conventionally associated with lions and eagles in medieval bestiaries is, in effect, ‘bracketed’: it is irrelevant for how the pictorial code actually functions.
The field is not always an undifferentiated, single tincture. It can also be divided in various conventional ways. For example, a shield divided ‘per bend’ has its field split in two diagonally, whereas a shield divided ‘per pale’ has its field split in two vertically.
The ‘rule of tincture’ governs the way in which tinctures may be assigned to the various layers of a shield. It states that a colour cannot be laid directly on top of another colour; nor can a metal be laid directly on top of a metal. So if the field is a metal, any ordinary laid on top of it must be a colour, while any charge laid on top of that must in turn be a metal. There are, however, some exceptions to this rule, notably that it does not apply to subordinaries, or to divisions of the field. 
In heraldry, blue represents only the idea of blue, and red the idea of red, and it does not matter which specific shade of blue or red is used to embody that idea. Indeed, in the seventeenth century, when it became common practice to print compilations of coats-of-arms in black-and-white reproductions, a system of cross-hatchings was invented to represent the tinctures, and these cross-hatchings represent their respective associated pigments perfectly: that is, in heraldry, a regularly-distributed pattern of dots signifies the idea of gold just as adequately as any particular yellow or gold pigment can do.
As I have described it thus far, heraldry is a purely visual system, but – to return to our metaphor of computer code – it has a written analogue, whereby a description of a shield can be created, which serves as a programme, whose output is the visual representation of the shield.
According to Shaun Tan, an image only works insofar as it can't be reduced to a written description (an argument that recalls Robert Frank's insistence that a photograph should nullify explanation), but heraldry is based upon the opposite assumption. As such, it offers one possible way of exploring the relationship between word and image, which is both a recurrent theme and a practical challenge in Five Wounds.[Continues in the next post:]
 ‘Dexter’ actually designates the right-hand side of the shield, and ‘sinister’ the left-hand side, but these are determined from the point-of-view of the fictional person holding the shield out in front of them towards the viewer. I have therefore simplified matters by sticking to the viewer's point-of-view here. This account of heraldry is also simplified in other respects.
 It does not apply to divisions of the field in theory. In practice, I have never seen a divided field that does not alternate a metal and a colour for the division.