Note 2 — PART III
Reading edition
Genitives and ablatives of description are adjective phrases. When we use an adverbial phrase to tell how long or how high or how deep anything is, we must use the accusative of extent. (Cf. § 336.) For example, in the sentence above multa mīlia passuum is an adverbial phrase (accusative of extent) modifying longam. If we should omit longam and say a fortification of many miles, the genitive of description (an adjective phrase) modifying mūnītiōnem would be used, as mūnītiōnem multōrum mīlium passuum.