Flakes may be produced by a variety of means. Force may be introduced by direct percussion (striking the core with a percussor such as a rock or antler), indirect percussion (striking the core with an object, sometimes referred to as a "punch," which itself is struck by a percussor, similar to the use of a hammer and chisel to shape stone), or by pressure. Additionally, flakes may be initiated in a Hertzian, bending, or wedging fashion. When a flake is detached from its core in a Hertzian fashion, the flake propagates in a conchoidal manner from the point of impact or pressure, producing a (usually) partial Hertzian cone. The cone of force often leaves a distinctive bulb of applied force on the flake and a corresponding flake scar on the core. A bending initiation results when a flake initiates not at the point where the force was applied, but rather further away from the edge of the core, resulting in a flake with no Hertzian cone or bulb of applied force and few if any of the characteristics ripples or undulations seen on the ventral surface of conchoidally produced flakes. Wedging initiation is the result of a strong hammer blow. At impact, concenctric radii emanate from the point of percussion, but unlike conchoidal fracture, the force travels along what would be the center of the Hertzian cone. The bipolar reduction technique is typified by its use of wedge initiation. Like bending initiation, no bulb of applied force results from wedging initiation, although in the bipolar technique, flakes may appear to have two points of percussion, on opposite ends, due to the fact that the core has been fractured by a hammer and anvil technique. The core is placed on a hard surface or "anvil" and is struck above by a hammer, thus the fracture may propagate from both ends simultaneously.
The end which received the blow or pressure is referred to as the proximal end of the flake; the terminal end is referred to as the distal end. The side displaying the bulb of force but without flake scars (barring an errailure flake scar or additional working of the flake) is called the ventral (or interior) surface, while the opposite side, displaying the flake scars of previous removals, or the cortical or original rock surface, is the dorsal (or exterior) surface.
On most natural cobbles or nodules of source material, a weathered outer rind called a cortex covers the unweathered inner material. Flakes are often differentiated by the amount of cortex present on their dorsal surfaces, because the amount of cortex indicates when in the sequence of reduction the flake came from. Primary flakes are those whose dorsal surfaces are entirely covered with cortex; secondary flakes have at least a trace of cortex on the dorsal surface; and tertiary (interior) flakes lack cortex, having derived entirely from the interior of the core. Primary flakes and secondary flakes are usually associated with the initial stages of lithic reduction, while tertiary flakes are more likely to be associated with retouching and bifacial reduction activities.
Prominent bulbs of force generally indicate that a hard hammer percussor (hammerstone) was used to detach the flake; flakes displaying this characteristic are referred to as conchoidal flakes. Hard hammer flakes are indicative of primary reduction strategies (e.g., core reduction, roughing of blanks and preforms, and the like). More moderate and diffuse bulbs may indicate the use of a soft hammer percussor -- such as bone, wood, or antler -- which produces the bending flakes often associated with bifacial thinning and trimming. The relative abundance of each type of flake can indicate what sort of lithic work was going on at a particular spot at a particular point in time.
A blade is defined as a flake with parallel or subparallel margins that is usually at least twice as long as it is wide. There are numerous specialized types of blade flakes. Channel flakes are characteristic flakes caused by the fluting of certain Paleo-Indian projectile points; such fluting produced grooves in the projectile points which may have facilitated hafting. Prismatic blades are long, narrow specialized blades with parallel margins which may be removed from polyhedral blade cores, another common lithic feature of Paleo-Indian lithic culture. Prismatic blades are often triangular in cross section with several facets or flake scars on the dorsal surface.
Secondary and tertiary flakes display dorsal flake scars, which are simply the markings left behind by flakes detached prior to the detachment of the subject flake. These flake scars are one of the lines of evidence used to infer the method of lithic reduction, or the process by which raw material is turned into useful objects.