[W]hen as a Gentleman of coat-armor hath married an heir to a Gentleman of coat-armor, and hath issue by her, that issue as heir, beareth the Arms of his father and of his mother in his Shield quarterly, and it is called Coat quartered plain, or rather, a Shield quartered plain.

– From Lacies Nobilitie by Sir John Ferne (1586), p102

Ferne also goes on to describe arms diverse quarterly, where different ancestral coats are displayed in each quarter, and arms quarterly quartered, which would occur when two people with arms quartered plain marry and have children. The configuration described here is by far the most common.

Quartering of arms is the evidence of maternal descent, and of the extinction of the immediate ancestors of the mother whose son becomes their heir general.

-From Inquiries into the Origin and Process of the Science of Heraldry in England by James Dallaway, p402

Before pedigrees were in use, the connection and descent of the bearer were illustrated by the insertion of more than one coat of arms into his escocheon, and the distribution and arrangement of these is termed marshalling.

-From Inquiries into the Origin and Process of the Science of Heraldry in England by James Dallaway, p397-8

When a female was heir of her mother and not of her father, who had male issue by a second marriage, he marshalled the maternal coat with the paternal upon a canton or in chief.

-From Inquiries into the Origin and Progress of the Science of Heraldry in England by James Dallaway, p341

Seal of William de Fortibus

1242

From Inquiries into the Origin and Process of the Science of Heraldry in England by James Dallaway, p445

The arms on the dexter side of the shield are those of de Fortibus, gules a cross patonce vair, and those on the sinister are or a lion rampant azure, the paternal arms of his wife, Isabella de Fortibus, nee Redvers. His arms are dimidiated while hers are impaled.

 

Even the ladies were as well versed in marshaling their hereditary achievement, as in the service of oratory.

-From Inquiries into the Origin and Process of the Science of Heraldry in England by James Dallaway, p137-138

The use of arms was closely connected with the study of genealogy; and when the mode of including in the same escocheon the armorial bearing of every heir female with whom an intermarriage had been made, was universally followed, they were the more necessary to each other.

-From Inquiries into the Origin and Process of the Science of Heraldry in England by James Dallaway, p136-7