A portion of the text (locus) in which the various witnesses display significant errors (cf. also variant location). The choice of a number of loci critici (selecti) allows the editor to establish a stemma in traditions that are hardly assessable in their entirety due to their width.
A recent coinage from Lat. Latin locus "passage, portion (of a text)" and criticus in the modern sense of "used for interpreting practice"; the (optional) adjective selectus alludes to the fact that the passage has been chosen for critical purposes. Though the expression seems to be first attested in the 1970s (see Balduino 1979), both Maas (first ed. 1927) and Pasquali (1952, first ed. 1934) had already introduced the similar notion of "collation by samples".
The most important editions of Dante's Divine Comedy are based on the scrutiny of loci critici: 396 of them from 200 witnesses in Barbi's preparatory work (known as "Barbi's canon"), and 477 in Petrocchi's edition (for further information see Brandoli 2007); in his 2001 edition of the Comedy, Federico Sanguineti goes back to Barbi's canon extending the collation to more than 500 witnesses.