代表空集,亦可代表圓的直徑。在電腦的部份字型以及早期的電報、無線電抄收時,會在數字0打上斜線(類似Ø)或是加入一個點以免與字母O混淆。 Pullum, Geoffrey K., & William A. Ladusaw. 1996. Phonetic Symbol Guide, 2nd ed. Chicago:
Longman. Rodney Huddleston & GeoffreyPullum. 2002. The Cambridge grammar of the English language. Cambridge, CUP. Huddleston和Pullum(頁數209-10)的主要論點是英語沒有未來時
Introduction to Historical Linguistics. 3rd edition. Oxford University Press. Pullum, Geoffrey K.; Ladusaw, William A. Phonetic Symbol Guide. University of Chicago