To provide for contrast and visibility, metals (generally lighter tinctures) must never be placed on metals, and colors (generally darker tinctures) must never be placed on colors. Where a charge overlays a partition of the field, the rule does not apply.
The field of a shield in heraldry can be divided into more than one tincture, as can the various heraldic charges. Many coats of arms consist simply of a division of the field into two contrasting tinctures. Since these are considered divisions of a shield the rule of tincture can be ignored. For example, a shield divided azure and gules would be perfectly acceptable. A line of partition may be straight or it may be varied. The variations of partition lines can be wavy, indented, embattled, engrailed, nebuly, or made into myriad other forms.
Two or more coats of arms are often combined in one shield to express inheritance, claims to property, or the occupation of an office. Marshalling can be done in a number of ways, but the principal mode is impalement, which replaced the earlier dimidiation which simply halves the shields of both and sticks them together.