The Diccionario de la prosa castellana del Rey Alfonso X, is the result of a multi-decade research project undertaken by a team of lexicographers at the Seminario de Estudios Medievales Españoles at the University of Wisconsin-Madison (USA) under the direction of Professors Lloyd A. Kasten and John J .Nitti. The DPCAX has its roots in 1935, in the pioneering work of Antonio García Solalinde and Lloyd A. Kasten, who began the creation of a lexicographic database with examples from the Alfonsine vocabulary taken from the transcriptions they had made for their edition of the General Estoria. To these, they added other transcriptions of chronicle, scientific and legal texts. After Solalinde's premature death, the direction of the Seminario was left in the hands of Professor Kasten, who decided to expand the scope of the project to try to compile an exhaustive dictionary of the medieval Spanish language. After the publication, in 1946, of the Tentative Dictionary of Medieval Spanish, the team dedicated itself to incorporating new material into the dictionary file until 1971. The second edition of the Tentative Dictionary, greatly expanded, was eventually published in 2001.

In the same year of 1971, the decision was made to abandon the project in its original form, and to take advantage of the possibilities offered by advances in the world of computing for the creation and manipulation of a vast database. The new project was called Dictionary of the Old Spanish Language (DOSL), of which DPCAX is one of its components.

The DPCAX is based on the lexicon of the texts that have been considered to have emanated from the royal chamber of Alfonso X, King of Castile and León between 1254 and 1284. They are, therefore, for the most part absolutely reliable texts, which offer special linguistic interest for constituting the first attempt to write an extensive corpus of texts in a standardized vernacular and supervised, at least in part, by the Wise King himself.

To avoid a fundamental defect in the methodology used in the creation of the Tentative Dictionary of Medieval Spanish, that of using as raw material later editions of medieval texts that are usually based on very diverse and, in some cases, very questionable, editorial criteria, it was decided to retranscribe all the selected texts following a very conservative transcription system developed by the DOSL team (and published in A Manual of Manuscript Transcription for the Dictionary of the Old Spanish Language), thus guaranteeing the rigorous documentation of the very rich lexical variation, both graphemic and morphological, that the medieval language offers. The printed version of the DPCAX was published in 2002, three years after the death of Professor Kasten, who dedicated most of his life to the creation of this dictionary and who spent the last years of his life correcting the galleys.

The online version of the DPCAX was possible thanks to the recovery of the files, in LaTex format, used for the typesetting of the dictionary. The lexicographic data (lemmas, grammatical information, definitions and examples) extracted from these files were subsequently processed with LexiquePro to generate the HTML files. Therefore the contents of this new version of the DPCAX do not differ from those that appear in the printed version.

Each of the DPCAX entries is headed by the lemma corresponding to the modern (etymologically related) form of the word if it exists. The grammatical category is indicated below, followed by the meanings, documenting for each of them all the variants, both graphemic and morphological. Each of the examples appears preceded by the abbreviation that identifies each of the sources and the folio and line where the defined term is found. Pluriverbal units, if they exist, occupy their own section at the bottom of the entry. The entry ends with a section listing the attested forms. Navigation within the dictionary is facilitated with the use of véase and véase también cross-references. The online version of DPCAX also allows access to lists of lemmas and forms, and the possibility of searching within them.