The constant gases in the atmosphere are those that remain relatively consistent in concentration over time and location. They are:

* Nitrogen (N2): This makes up the largest portion of the atmosphere (about 78%).

* Oxygen (O2): The second most abundant gas (about 21%).

* Argon (Ar): Makes up about 0.93% of the atmosphere.

These gases are considered constant because their concentrations don't fluctuate significantly over time or place.

It's important to note that while these gases are considered constant, their concentrations can vary slightly due to factors like altitude, geographical location, and human activities.