Consider the humble tally. Whether it is made of notched wood, knotted string, a torn leaf, strung beads, loose pebbles, marks painted on the body or inscribed on the ground, the fingers, the fingers and toes, or the fingers plus other body parts, a tally is a simple device, as material forms go, one that requires few resources to learn or invent from scratch. But because it is a material form that is not a part of the body, the tally represents an extremely powerful mechanism – the ability of the material form to accumulate and distribute cognitive effort – that for numbers begins with the tally and continues today with calculators and computers. If the tally is easy for a novice to understand, use, make, and invent, a device like the computer is not, even for an expert. This is because at some point, the amount of cognitive effort needed exceeds what a single individual, or even an entire generation of people, can manage on its own. Material devices also have a capacity for manipulability and morphological change that far exceeds what bodies and behaviors are capable of; they are also public and shareable in ways that bodies and behaviors are not. The tally thus represents a significant step in harnessing the agency of material forms toward numerical purposes.