I have looked at the Wikipedia article "Couplet (Chinese Poetry)", but I think it poses more questions than answers.
So a 对联 (duì lián) is a "Chinese couplet":
- it is composed of two lines;
- each line must have the same number of characters;
- the characters on each line must have a correspondence with each other;
- the characters on each line must have opposite "tone pattern" (平 vs 仄).
If a 平 tone is a "first tone", and all other tones are 仄, then the Wikipedia example doesn't make sense (qín wéi is 平?).
And I'm unclear as to whether each "character" must be related, or each "word"?
For example, 很贵 and 便宜 have opposite meanings as words, but not as characters.
As an example, is this a 对联?
猪扒便宜 Zhū pá piányi
鸡腿很贵 Jītuǐ hěn guì