The spelling of the phrase "most back number" is fairly straightforward when using IPA phonetic transcription. The word "most" would be transcribed as /moʊst/, while "back" is /bæk/ and "number" is /ˈnʌm.bər/. When combined, the phrase is pronounced as /moʊst bæk ˈnʌm.bər/. This phrase is often used to describe the oldest or least desirable item in a group, often in reference to a collection of items such as books or magazines.