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Long anisometric molecules cannot pack independently. In the Onsager excluded-volume mechanism, two slender rods of length $L$ and diameter $D$ exclude a relative-position volume approximately $2L^{2}D|\sin \gamma |$, where $\gamma$ is their mutual angle. Aligning rods reduces this excluded volume, increasing the translational entropy available at a fixed number density. It simultaneously reduces orientational entropy. At sufficiently high concentration the positional gain wins, producing a nematic liquid crystal even for purely repulsive rods.

The nematic director is apolar: $\mathbf{n}$ and $-\mathbf{n}$ describe the same orientation. The molecules retain liquid-like positional disorder, in contrast to a smectic phase with layered order. A typical dimensionless concentration controlling the slender-rod competition is $c{L}^{2}D$, although finite aspect ratio, flexibility and attractions change the transition. The mechanism illustrates that ordering can increase total entropy: the lost orientational freedom is compensated by less mutual obstruction.