The Marcher Lords were those lords placed in possession of the counties on the border between Wales and England after the Norman Conquest. One was the Earl of Hereford, the second, the Earl of Chester, and our family was subject to Montgomery the Earl of Shrewsbury. This would be the present day area of Shropshire. Montgomery is a town inside Wales in Montgomeryshire.
Basically these great men(Magnates) were more or less independent of the king, acting much like kings themselves. They dispensed justice and ruled the area in every way independent of the king except for treason cases.
Sitting immediately adjacent to Montgomeryshire, inside Shropshire, was Owestry, an early seat of the Corbets, and wholly inside England, despite its rule by Montgomery. However it was some time before this border line between Wales and England at the the break between the two counties was drawn. There are Dikes or earthworks in the area of Owestry which mark earlier 9th century and earlier borders with Wales.
For various and sundry reasons, most of the rights and lands of these Marcher lords came back under the control of the king in just a few generations. The grants of land became vacant through lack of an heir, the Earls proved to be less than loyal, etc..