Why Are There No Relational DBMSs? by Hugh Darwen (2015).

Textbook Treatments of Relational Algebra by Hugh Darwen and C.J. Date (2015).  A companion paper to Why Are There No Relational DBMSs

Codd's First Papers: A Critical Analysis by C.J. Date (2015).  Another companion paper to Why Are There No Relational DBMSs.

Observations on the 1969 Relational Operations by David McGoveran (2005).  A paper in similar vein to Date's Codd's First Papers: A Critical Analysis.  McGoveran discusses Codd's very first paper on relations.  

An Algebra of Relations for Machine Computation by Patrick Hall, Peter Hitchcock, and Stephen Todd (1975) annotated in 2015 by Hugh Darwen, in whose opinion this was "the paper that should have been the game-changer", showing the right way forward with the relational model.

Feature Analysis of Business System 12 by Ghica van Emde Boas (a member of the BS12 development team, 1979-1986)

The Influence of ISBL by Hugh Darwen, a comparison table showing how ISBL influenced the sematics of relational operators in Business System 12 and Tutorial D.  Thanks to Stephen Todd for help with this one.

Could packing be specified in terms of closed-open interval semantics without having to commit to granularity? by Erwin Smout, referenced in Time and Relational Theory.

Extending Tutorial D to Support User-Defined Generic Relation and Tuple Operators by Hugh Darwen, a tentative proposal in response to a requyirment that is frequently rised in TTM discussions.  Comments are invited in the TTM forum.

Headings and Bodies by Hugh Darwen, a contribution to a debate going on in the TTM forum, October 2013.

The Relational Model by Hugh Darwen (to accompany his article The Relational Model: Beginning of an Era, included in the autumn 2012 special issue of the IEEE Annals of The History of Computing).

All for One, One for All by C.J. Date (on relationships and their properties)

Reactions to "A Call to Arms" (an article that made our knees jerk)

Towards an Agreeable Model of Type Inheritance by Hugh Darwen

An Overview and Analysis of TSQL2 by Hugh Darwen and C.J. Date

How To Handle Missing Information Using S-by-C by Hugh Darwen and Erwin Smout (review draft). This approach is described again and discussed in Chapter 25, "An Inheritance Approach", of Database Explorations, which was published in 2010. From the same book, Chapter 24, The Multirelational Approach (to the problem of missing information) is a joint effort by Chris Date and Hugh Darwen, based on and now superseding Darwen's 2008 review draft Multirelations.

Four related papers by C.J. Date:

A Discussion of Certain Criticisms of The Third Manifesto (introduces the next three)

Gödel, Russell, Codd: A Recursive Golden Crowd

And Now for Something Completely Computational

To Be Is to Be a Value of a Variable