A number of verbs have fewer than four principal parts: deponent verbs, such as "hortŏr – hortāri – hortātus sum", "to exhort", lack a perfect form, as do semi-deponent verbs, such as "audeō – audēre – ausus sum", "to dare"; in both cases, passive forms are treated as active, so all perfect forms are covered by the perfect participle. A handful of verbs are also defective, including the verb "ōdī – ōdisse", "to hate", which only has perfect forms derived from a single stem.