One-to-one is used when you talk about transfer or communications. You may use one-to-one when you can identify a source and a destination. For eg., a one-to-one email is one sent from a single person to another, i.e., no ccs or bccs.

Understanding the Context

In maths, a one-to-one mapping maps one element of a set to a unique element in a target set. One-on-one is the correct adjective in your example. See Free ... Both 'a single' and 'one single' are correct and commonly used phrases in English.

Key Insights

They are interchangeable and can be used to emphasize the singularity of an item or person. IFLScience: The Average Person Holds Nine "Moderately Significant" Secrets, And They're Roughly What You'd Expect The Average Person Holds Nine "Moderately Significant" Secrets, And They're Roughly What You'd Expect The 'one hand' could be any generic hand (no article) or a specific one (definite article). 'The other' refers to the 'matching hand' of the first one. There can only ever be one, and it is being referred to explicitly as the partner of the first, therefore it needs a definite article. MSN: The average person has nine deep dark secrets, study reveals - so, are yours eating you alive?

Final Thoughts

The average person has nine deep dark secrets, a new study has revealed. Researchers from the University of Melbourne asked 240 people about the things they keep hidden from others. The results ... The average person has nine deep dark secrets, study reveals - so, are yours eating you alive?