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In our solar system, Mercury and Venus are inferior planets: their orbits are entirely inside the Earth's orbit. When seen from the Earth, inferior planets go through phases, like the Moon's.

An inferior planet on the same side of the sun as the Earth appears dark and is not easy to see. Near the opposite side of the sun, we see the planet nearly in full illumination, and in between, we see crescents of various sizes.

Snapshot 1: Mercury and Venus are both in a crescent phase.

Snapshot 2: Mercury and Venus are both in a first-quarter phase.

Snapshot 3: Venus (left) is in a crescent phase and Mercury (right) is in a last-quarter phase.

Snapshot 4: Venus is in a full phase and Mercury is in a first-quarter phase.

Snapshot 5: Viewed from Mercury, neither the Earth or Venus can be viewed as a crescent since they are both in superior orbits.

Snapshot 6: Viewed from Venus, only Mercury can be seen as a crescent. Earth shows only minor phase changes.