Matching patterns with variables -- Abelian properties of words -- On sets of words of rank two -- On families of limit S-adic words -- Independent systems of word equations: from Ehrenfeucht to eighteen -- Parikh determinants -- Critical exponent of infinite balanced words via the Pell number system -- Repetitions in infinite palindrome-rich words -- Generalized Lyndon factorizations of infinite words -- On the commutative equivalence of bounded semi-linear codes -- Circularly squarefree words and unbordered conjugates: a new approach -- The undirected repetition threshold -- Characteristic parameters and special trapezoidal words -- Return words and bifix codes in eventually dendric sets -- Enumeration and extensions of word-representable graphs -- Localisation-resistant random words with small alphabets -- On codeword lengths guaranteeing synchronization -- Binary intersection revisited -- On substitutions closed under derivation: examples -- Templates for the k-binomial complexity of the Tribonacci word -- Derived sequences of Arnoux-Rauzy sequences -- New results on pseudosquare avoidance -- Every nonnegative real number is a critical abelian exponent -- Rich words containing two given factors -- Mortality and synchronization of unambiguous finite automata -- On discrete idempotent paths.

This book constitutes the refereed proceedings of the 12th International Conference on Combinatorics on Words, WORDS 2019, held in Loughborough, UK, in September 2019. The 21 revised full papers presented in this book together with 5 invited talks were carefully reviewed and selected from 34 submissions. WORDS is the main conference series devoted to the mathematical theory of words. In particular, the combinatorial, algebraic and algorithmic aspects of words are emphasized. Motivations may also come from other domains such as theoretical computer science, bioinformatics, digital geometry, symbolic dynamics, numeration systems, text processing, number theory, etc.