The Plain of Mars is the hollow at the centre of the palm, between the two Mars mounts and below the other mounts, and it is not a mount at all but the depression they surround. The tradition reads its depth and firmness for the general temper of the person, a deep and soft plain indicating sensitivity and a shallow or hard one resistance, and treats it as the field across which the principal lines run. Because the lines cross it, much of the detailed reading in this school happens here. The plain is also the clearest illustration of the method: the anatomy of the palm supplies a set of natural regions, the scheme assigns each a planet or a quality, and interpretation proceeds by combining them. What the method never does is test whether the assignments correspond to anything.
The Plain of Mars is not a raised mount but the flattened or slightly hollowed central area of the palm, bounded by the mounts of Jupiter, Saturn, Apollo and Mercury above and Venus, Luna, and the Upper and Lower Mounts of Mars around its edges. Traditional chiromancy reads its depth and quality as a general indicator of a person's underlying vitality and resilience, a hollow, deeply cupped plain suggesting a tendency toward being easily overwhelmed by circumstance, while a level, well-filled plain suggests steadiness under pressure. Because the plain sits at the meeting point of all the surrounding mounts, some manuals treat it as a summary field, reflecting the balance or imbalance among the other seven mounts, rather than as an independent indicator with a fixed meaning of its own.