因為這里的不定冠詞是修飾名詞cottage,而不是修飾序數。又如:
The film is a costume drama based on a 19th-century novel. 這部電影是根據一本19世紀的小說拍成的古裝劇。
It is a 16th-century church with 18th-century embellishments. 那是一座有18世紀藝術裝飾的16世紀的教堂。
如果名詞中心詞前無需用冠詞,則此時修飾名詞的序數詞前也不能用冠詞。如:
The play is set in 15th-century Italy. 這出戲以十五世紀的意大利為背景。