Robert G. Natelson (The Independence Institute & Montana Policy Institute) has posted
The Origins and Meaning of 'Vacancies that May Happen During the Recess' in the Constitution's Recess Appointments Clause on SSRN. Here is the abstract:
There has been longstanding uncertainty about the meaning of “the Recess” and “Vacancies that may happen” in the Constitution’s Recess Appointments Clause. This Article finds that both “the Recess” and close variants of “Vacancies that may happen” were standard terms in Founding-Era legislative practice, and appear copiously in legislative records. Those records inform us that “the Recess” means only the inter-session recess and that a vacancy “happens” only when it first arises.
Very interesting & essential for those interested in this issue.