# Family contact

## The contract in words, and how to choose

No hand-written part: how to choose among this family's laws, or to attach them together, is not written yet.

## Contract

What every member must satisfy, as the family's harness checks it on the member's own node.

- Sign rule: where the body is projected back, it moves no further out
- Cases a member passes: `sign`, `frame`, `units`, `material reach`, `oracle`, `nodes`
- Settings that stand for a material's properties, each of which must reach the output: `extent`
- Deliveries a two-way law makes equal and opposite: none
- Reference member a swap replaces: none
- Settings proportional to a material property: none

## Members

| Entry | Rung | Derivative |
|---|---|---|
| [`contact`](../entries/interaction/contact.md) 0.4.0 | [`projection`](../entries/interaction/contact.md#rung-projection) | `piecewise_jump` |

- reserved, not built: [`penalty`](../entries/interaction/contact.md#rung-penalty) of `contact`: penalty contact, delivering loads into each side's applied wrench, comes later
- reserved, not built: [`penalty`](../entries/interaction/contact.md#rung-penalty) of `contact`: penalty contact, delivering loads into each side's applied wrench, comes later

## Comparison

This family has one built rung, so there is nothing to compare.