“Chalk and cheese” is an idiom used when two things are completely different, even if someone first compares them. It is often used for people, jobs, ideas, products, or experiences.
The likely origin is practical and visual. In old market settings, chalk and cheese could appear similar in color, but they were entirely different in substance, value, and purpose. That contrast helps explain the modern meaning: things may look alike at a glance, but in reality they are nothing alike.
You will often see the expression after a comparison:
- The two sisters are chalk and cheese in temperament.
- City life and farm life are chalk and cheese.
- Our old software and the new system are chalk and cheese.
This idiom is common in British English and is less common in American English, where people may simply say very different or totally different. Still, the meaning is easy to understand once you know the contrast behind it.
Use it when you want a vivid, informal way to say that two things differ sharply, not just in one small detail, but in their basic nature.