The most important contribution made by Archbishop James Ussher to the ecclesiastical developments of the Interregnum and Restoration periods was his short tract The Reduction of Episcopacy Unto the Form of Synodical Government. Printed only after his death in 1656, its combination of ministerial synods with episcopal rule was seen as a basis for presbyterian-episcopal reconciliation over the next three decades. The tract was printed in five editions during the later 1650s, and came out in two more editions in 1679, when the Popish Plot and the calling of a new Parliament revived hopes that dissenters could be comprehended within the Church of England. It was printed once more in 1689, in Edinburgh, when “comprehension” was again being hotly debated in both England and Scotland. By that time Ussher's name had come to symbolize such “limited” or “primitive” episcopacy, and indeed it has continued to do so among twentieth-century historians.
The fame of the Reduction rests upon its content and authorship. Although the tract was only one of many such compromises offered during the Interregnum, it was the most radical to come from the royalist and Anglican side during that period. Archbishop of Armagh and Primate of all Ireland, Ussher was admired and respected by radical puritans and major Laudian spokesmen such as Henry Hammond and Bishop John Bramhall. The power of Ussher's name in this context was shown in 1685, when the nonconformist divine and politician Richard Baxter was on trial for allegedly making a printed attack against the king and the bishops. When Baxter's attorney, Sir Henry Pollexfen, sought to introduce as evidence one of Baxter's own printed compromises between episcopal and presbyterian government, Lord Chief Justice George Jeffreys replied, “I will see none of his books; it is for primitive Episcopacy, I will warrant you — a bishop in every parish.” In replying “Nay, my lord, it is the same with Archbishop Usher's,” Pollexfen indicated both the radical nature of the Reduction and the legitimacy that Ussher's name lent to other compromises of this kind.