From a number of Czech adjectives, both an adverb with the suffix -o and an adverb with the suffix -e are derived, cf. deštivý `rainy’ – deštivo/deštivě, drahý `expensive’ – draho/draze. These pairs of adverbs have often the same meaning but the adverb with the suffix -o occurs as a part of the predicate (with the verb být `to be’; ráno bylo deštivo `in the morning it was rainy’) while the adverb with the suffix -e is often specialized for the adverbial function (e.g. ráno vypadalo deštivě/*deštivo `the morning looked rainy’).
On the functional specialization Komárek’s (1954) proposal to separate the adverbs with the suffix -o as an autonomous part of speech was based. Since the actual corpus material indicates that both the adverb with the suffix -o and the adverb with -e are acceptable in the same context (cf. je tam draho/draze `it is an expensive place to live’, draho/draze prodat `to sell dear’ VS. přišlo ho to draho/*draze ` it cost him dear’, draze/*draho za to zaplatil `he paid for this’) but the (non)acceptability cannot be probably explained by a grammatical principle, it seems to be appropriate to consider these words further as adverbs and to describe them as separate lexical units rather than a homogenous group.