The name "Diez" itself seems to be a corruption of the Frankish Theodissa, which evolved to Diedisse, which mutated over time to Dietz and finally to today's spelling of Diez.
 The settlement was known as Theodissa by the year 790 A.D. according to the charter of Charlemagne where it is listed as property of the Abbey of Prüm.
Other early mentions In the post-Carolingian period include a reference to one Diez in the area of Niederlahngau, ruled by the Conradines.

In 1073 that the Count of Die(t)z was mentioned for the first time in a deed of sale for goods in Bodenheim.
Early references show that Heinrich II von Diez (1145–1189) accompanied Frederick Barbarossa on his Italian campaign where he was involved in diplomatic negotiations.
His son Henry III was part of the regency council and circle of tutors of for Henry VII, which seems to indicate an atmosphere of growing influence for the town.
The Graf Gerhard IV (1276–1308) founded in 1289 a collegiate at the foot of the castle hill.
The founding community came from the Abbey of Salz.
The monastery was named Saint Mary's after the patron saint of the church.
 The town continued to prosper.
 By 1329, Ludwig of Bavaria bestowed the right of municipal law to Diez, and at that time the town was fortified with a wall with five entry gates.

The dynasty of the Counts of Diez ended in 1386 with Gerhard VII.
His daughter Jutta of Diez had married Adolf of Nassau (1362-1420) in 1376 and inherited the county.
Their only daughter, Jutta of Nassau, married Godfrey of Eppstein and inherited half of the county while the other half came to Adolf's brother Engelbert I of Nassau.
The latter's descendants, the counts of Nassau-Dillenburg, acquired further parts of the Eppstein share in 1530, parts that had come down to the Counts of Katzenelnbogen and, after they died out in 1479, to Landgraf Heinrich III of Hesse.
From 1564 on, Diez castle was solely owned by Nassau-Dillenburg.
The name "Diez" itself seems to be a corruption of the Frankish Theodissa, which evolved to Diedisse, which mutated over time to Dietz and finally to today's spelling of Diez.  The settlement was known as Theodissa by the year 790 A.D. according to the charter of Charlemagne where it is listed as property of the Abbey of Prüm. Other early mentions In the post-Carolingian period include a reference to one Diez in the area of Niederlahngau, ruled by the Conradines. 
In 1073 that the Count of Die(t)z was mentioned for the first time in a deed of sale for goods in Bodenheim. Early references show that Heinrich II von Diez (1145–1189) accompanied Frederick Barbarossa on his Italian campaign where he was involved in diplomatic negotiations. His son Henry III was part of the regency council and circle of tutors of for Henry VII, which seems to indicate an atmosphere of growing influence for the town. The Graf Gerhard IV (1276–1308) founded in 1289 a collegiate at the foot of the castle hill. The founding community came from the Abbey of Salz. The monastery was named Saint Mary's after the patron saint of the church.  The town continued to prosper.  By 1329, Ludwig of Bavaria bestowed the right of municipal law to Diez, and at that time the town was fortified with a wall with five entry gates. 
The dynasty of the Counts of Diez ended in 1386 with Gerhard VII. His daughter Jutta of Diez had married Adolf of Nassau (1362-1420) in 1376 and inherited the county. Their only daughter, Jutta of Nassau, married Godfrey of Eppstein and inherited half of the county while the other half came to Adolf's brother Engelbert I of Nassau. The latter's descendants, the counts of Nassau-Dillenburg, acquired further parts of the Eppstein share in 1530, parts that had come down to the Counts of Katzenelnbogen and, after they died out in 1479, to Landgraf Heinrich III of Hesse. From 1564 on, Diez castle was solely owned by Nassau-Dillenburg.
Diez, Germany