futari wrote:
"one-to-one"

a) A *relationship* in which someone deals with only one other person.
[= (關係)一對一的; 面對面的/當面的(如: 諮詢, 教授等]
e.g. Children benefit from one-to-one adult attention.

b) A *comparison* in which something is compared with something else that is broadly equivalent or similar to it.
[= 對等的; 相應的]
e.g. These terms are not one-to-one translatable.

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"one-on-one"

a) (as adjective) Consisting of or being direct communication or exchange between two individuals, especially in a *conflict*.
[= 一打一的; 面對面的]

b) (as noun) Being a system of play in *sports* in which an individual defensive player guards an individual offensive player.
[= 人釘人; 一打一/一對一(球賽), 尤指兩人間的非正式球賽]