KETER
References from Jewish Mysticism

The Crown image of the Merkavah is as relevant as the Throne and Chariot. They are interchangable. It relates directly to the Great Angel Measurement, the wearer of the Crown, who defines the outer limits of the Sanctuary.

Excerpts from Keter: The Crown of G-D in early Jewish Mysticism
Arthur Green
1997
Princeton University Press

With notes in bold my interpretation

Introduction

The most pragmatic of all references to divine Kingship in the regular weekly liturgy is in the introduction to the "Qedushah"... as recited by Sephardic Jews in the "Mussaf" service on Sabbaths, New Moons and Festivals.

"A Crown they give unto you, YHWH our G-D, angels enthroned above and your people Israel gathered below. All of them thrice proclaim your holiness, as has been spoken by the prophet's word: "They call out to one another, saying: "Holy, Holy, Holy is the Lord of Hosts! The whole earth is filled with His Glory!"

In the liturgy Israel recounts an enthronement rite that takes place daily in the upper realms. But in the "Qudushah" of the Amidah repetition, which requires a quorum of ten for public recitation, Israel rises to join in with the angels in bestowing the crown upon the divine head, the joint gift to G-D from His loyal legions above and below. "Dwellers in the upper and lower realms."