In the semiotic theories of Charles Sanders Peirce, an index is a type of sign that signifies by having a direct connection to what it signifies – smoke is an index of fire, and a pointing finger is an index of what it indicates. The index is one of a trichotomy of sign types, the other two being the icon (which signifies by resemblance) and the symbol (which signifies by conventional association). Most semiotic constructions have elements of all three, and book indexes are no exception. The way signs are interpreted involves another trichotomy, of types of inference: abduction, deduction and induction. What readers take away from your index will depend on how you manage it – and your process of creating it – to optimize its indexicality, iconicity and symbolicity for optimal abduction.