1 Accepted for publication 21-Nov-2017 “This is an Accepted Manuscript of an article published by Routledge Taylor & Francis Group in [European Education] on [Published online: 01 Feb 2018], available online: https://doi.org/10.1080/10564934.2017.1411764” Horizontal differentiation matters: Moderating influence of the type of upper secondary education on students' transitions Pepka Boyadjieva Institute for the Study of Societies and Knowledge, Bulgarian Academy of Sciences 1000 Sofia, 13A Moskovska str., Bulgaria Tel: „00359“ – „899 751 574“ E-mail: pepka7@gmail.com Petya Ilieva-Trichkova 1 Institute for the Study of Societies and Knowledge, Bulgarian Academy of Sciences 1000 Sofia, 13A Moskovska str., Bulgaria Tel: „00359“ – „898 907 529“ E-mail: petya.ilievat@gmail.com 1 Equal authors, the names are listed in an alphabetical order. Corresponding author: Petya Ilieva-Trichkova, Institute for the Study of Societies and Knowledge, 13A Moskovska str., 1000 Sofia, Bulgaria. Email: petya.ilievat@gmail.com