from a number of sources including manuals and workshops, and obtain certification through testing. The original FACS has been modified to analyze facial movements in several non-human primates, namely chimpanzees, rhesus macaques, gibbons and siamangs, and orangutans. More recently, it was developed also for domestic species, including dogs, horses and cats. Similarly to the human FACS, the animal FACS has manuals available online for each species with the respective certification tests. Thus, the FACS can be used to compare facial repertoires across species due to its anatomical basis. A study conducted by Vick and others (2006) suggests that the FACS can be modified by taking differences in underlying morphology into account. Such considerations enable a comparison of the homologous facial movements present in humans and chimpanzees, to show that the facial expressions of both species result from extremely notable appearance changes. The development of FACS tools for different species allows the objective and anatomical study of facial expressions in communicative and emotional contexts. Furthermore, a cross-species analysis of facial expressions can help to answer interesting questions, such as which emotions are uniquely human. The Emotional Facial Action Coding System (EMFACS) and the Facial Action Coding System Affect Interpretation Dictionary (FACSAID) consider only emotion-related facial actions. Examples of these are: Codes for action units For clarification, the FACS is an index of facial expressions, but does not actually provide any bio-mechanical information about the degree of muscle activation. Though muscle activation is not part of the FACS, the main muscles involved in the facial expression have been added here. Action units (AUs) are the fundamental actions of individual muscles or groups of muscles. Action descriptors (ADs) are unitary movements that may involve the actions of several muscle groups (e.g., a forward‐thrusting movement of the jaw). The muscular basis for these actions has not been specified and specific behaviors have not been distinguished as precisely as for the AUs. For the most accurate annotation, the FACS suggests agreement from at least two independent certified FACS encoders. Intensity scoring Intensities of the FACS are annotated by appending letters A–E (for minimal-maximal intensity) to the action unit number (e.g. AU 1A is the weakest trace of AU 1 and AU 1E is the maximum intensity possible for the individual person). A Trace B Slight C Marked or pronounced D Severe or extreme E Maximum Other letter modifiers There are other modifiers present in FACS