The choice between "sitting at a desk" and "sitting on a desk" depends on the context and what you want to convey. Both phrases are correct, but they describe different scenarios.

Sitting at a Desk

When you say someone is "sitting at a desk," it typically means they are seated in front of a desk, usually working, studying, or performing some task. The focus is on the person being positioned near the desk, often using it as a workspace. For example:

• "She was sitting at her desk, typing an email."
• "He spends hours sitting at his desk, working on his novel."

This phrase implies a functional relationship between the person and the desk, where the desk serves as a tool or surface for activity.

Sitting on a Desk

When you say someone is "sitting on a desk," it means they are physically perched or seated on top of the desk itself. This might suggest a more casual or informal posture, often used in relaxed or conversational settings. For example:

• "The teacher was sitting on the desk while explaining the lesson."
• "He sat on the desk, swinging his legs as he chatted with his colleagues."

This phrase emphasizes the physical act of sitting on the desk, often implying a lack of formality or a break from traditional desk use.

Key Differences

1. Positioning: "At a desk" suggests being in front of or near the desk, while "on a desk" means physically on top of it.
2. Formality: "At a desk" is more formal and work-related, while "on a desk" is casual and relaxed.
3. Imagery: "At a desk" evokes images of productivity, while "on a desk" might suggest a pause or informal interaction.

Examples in Context

• At a desk: "The lawyer was sitting at his desk, reviewing the case files."
• On a desk: "The student sat on the desk, waiting for the professor to arrive."

Conclusion

Both phrases are correct, but they serve different purposes. Use "sitting at a desk" to describe someone working or studying, and "sitting on a desk" to describe someone sitting on top of the desk in a casual manner. The choice depends on the image you want to create in the reader's mind.