Field Plate Mail

Field plate is actually a more commonly used form of full plate mail. It consists of shaped and fitted metal plates riveted and interlocked to cover the entire body. Like plate mail, a set of field plate usually includes gauntlets, boots, and a visored helmet. A thick layer of padding must be worn under the armor.
This armor is rarely used, except by noble knights on a military campaign. In theory, the bulk of a set of field plate armor is so evenly distributed over the whole body that the encumbrance rating of field plate compares quite favorably with that of plate mail and banded mail. In practice, the increased protection is paid for with reduced mobility and increased fatigue.
Each suit of this extremely rare and expensive armor is custom-made and fitted for its prospective wearer. Only a master armorer can create field or full plate armor, and only a master armorer can re-size captured pieces of a suit for a new owner. The new owner must be of at least a similar size and build as the previous owner, or the effort required to modify the piece in question exceeds the expense and effort necessary in forging an entire new set.
Aside from its expense, the main disadvantages of field plate armor are the lack of ventilation through the suit, which make moisture and fungus a problem, and the time required to put it on and take it off. 1d6+4 rounds are required to dress in field plate armor with outside assistance. Triple that amount of time is required if the wearer is alone. Similarly, it takes 1d4+1 rounds to remove such armor, and half that time (fractions rounded up) if pressed for time or assisted by an attendant (see Full Plate Mail).
Field plate armor is typically used by the high knights of a kingdom, like King Arthur's legendary Knights of the Round Table, for everything except formal ceremonies and triumphant battle celebrations. Full plate armor is reserved for such occasions where style is more important than combat.
Field plate can be fixed much more cheaply and easily than full plate, and is built to withstand the rigors of long-term use and combat. The entire body is encased in metal plates, and even the joints are protected with metal caps and sturdy chain mail. A great helm bearing the emblem of the house or name of a legendary knight is common and is one of the few ways a knight can recognize a friend or foe from a distance.
Field plate, while expensive and painstakingly crafted by master armorers, is normally not adorned with many trappings or embellishments. Since legendary knights make a habit of battling great armies and dragons almost daily, their armor rarely lasts longer than a year without needing to be completely replaced.
A DM may decide that being allowed to wear field or full plate armor is a sign of nobility or knighthood in his campaign. In such campaigns, wealthy warriors had better have a legitimate noble crest or recognized royal patron before parading themselves about town.