1986: Peter V. Jones and Keith C. Sidwell, Reading Latin: Grammar, vocabulary and exercises, Introduction — General notes (note 1), page 2 (22ⁿᵈ printing (2007); Cambridge University Press; →ISBN All vowels are pronounced short unless marked with a ¯ (macron) over them. So observe different vowel length of ‘i’ in, e.g., fīlia, etc. It may be helpful, but is not essential, to mark macra in your exercises.
Source: wiktionary