↑Curta 2008, pp.350–351 harvnb error: no target: CITEREFCurta2008 (help)
↑Language Contact in Europe
The Periphrastic Perfect Through History by Bridget Drinka (2017) Cambridge University Press page 290 (ISBN: 9780521514934)
123Rashev, Rasho (2008). Българската езическа култура VII -IX в./Bulgarian Pagan Culture VII - IX c. (in Bulgarian). Класика и стил. ISBN9789543270392.
↑Davies, Norman (1997). Europe. A History. Oxford University press. ISBN954-427-663-7.
↑Radoev, Ivanov Alexander (2019). THE CROATIAN ORTHODOX CHURCH Was, Is, And Shall Be. Zagreb, Croatia: Croatian Archbishop Alexander. p. 64. ISBN978-0-359-99545-5. According to the law of 893, in the Bulgarian Empire everyone was Bulgarian, and the Bulgarian language was mandatory in the liturgy (with Cyrillic or Glagolitic script).