the hammer used to form the piece and the angle obtained etc.). The most commonly used coefficients were established by Bordes for the morphological-mathematical classification of what he called "classic bifaces" (Balout proposed other, similar indices): Base-rounding index—allows classic bifaces to be separated into triangular, almond-shaped and oval families. The L/a ratio provides the following separation thresholds: Elongation index—separates common bifaces from the short (and, occasionally, from the elongated). For example, within the family of Oval bifaces the index distinguishes discoidal bifaces from other types; in the Almond-shaped bifaces it identifies lanceolate or micoquien bifaces. The index is calculated using L/m. Thresholds: Cross-sectional/planar index—divides the thick bifaces from the flat ones and is used only on certain types. In the Almond-shaped bifaces (along with the elongation index) it distinguishes the amygdaloid bifaces (thick) from the cordiform bifaces (flat). The index is calculated using m/e. Thresholds: Other indexes apply to the other types of bifaces (partial bifaces, bifaces with an unworked base, or cleaver, spatulate, Abbevillean, nucleiform, etc.). Bordes hand axe typology The following guide is strongly influenced by the possibly outdated and basically morphological "Bordes method" classification system. This classification is particularly applicable to classic hand axes, those that can be defined and catalogued by measuring dimensions and mathematical ratios, while disregarding nearly all subjective criteria. "Distinguishing between different types of hand axes is not always easy. There is often no room for doubts, however, there are a number of cases where the difficulty is real." In the majority of cases, this system agrees with previously established categories (although slightly redefining them). Balout made a similar attempt at categorization. Non-classic specimens Many specimens defeat objective classification. Bordes created a group he called "non-classic bifaces" to which mathematical indexes do not apply. Nucleiform bifaces—It is difficult to distinguish a true biface from a core with modified edges that was occasionally used as a tool. A piece could also be a blank or a chance occurrence. Despite their rough appearance, nucleiform bifaces were present in both the Acheulean and the Mousterian. Cleaver-bifaces—These bifaces have an apex that is neither pointed nor rounded. They possess a relatively wide terminal edge that is transverse to the morphological axis. This edge is usually more or less sub-rectilinear, slightly concave or convex. They are sometimes included within the classic types as they have a balanced, well-finished form. Cleaver-bifaces were defined by Chavaillón in 1958 as "biface