Unitary demand curve: p = 1 - q
This demand function is just normalized to unit. If you are not interested in the concrete quantity of your demand function, but you are interested how the demand of the specific market is distributed between firms, you can use this simple demand function.

For example, you can consider a normalized demand function in a Bertrand's game. Let's assume the firms are symmetric, there's no capacity constraints. The demand ($D(p_i,p_{-i})$) of firm $i$ is the following:

$$ D(p_i,p_{-i}) =
\begin{cases}
    0       &amp; \quad \text{if } p_i &gt; p_{-i} \\
    1/2  &amp; \quad \text{if } p_i = p_{-i} \\
    1  &amp; \quad \text{if } p_i &lt; p_{-i} \\
  \end{cases} $$