A rectangle with the side lengths $a$ and $b$ with $a <b$ should be placed in a right-angled coordinate system so that there is no point with integer coordinates in its interior or on its edge.
Under what necessary and at the same time sufficient conditions for $a$ and $b$ is this possible?
1. **Understanding the Problem:**
   We need to place a rectangle with side lengths \(a\) and \(b\) in a right-angled coordinate system such that there are no points with integer coordinates inside or on the edge of the rectangle. 

2. **Initial Consideration:**
   If both \(a\) and \(b\) are \(\geq 1\), then it is impossible to place the rectangle without having integer coordinate points on its edge or inside it. This is because the distance between any two integer points is at most 1, and a rectangle with both sides \(\geq 1\) will span at least one unit in both directions.

3. **Condition for \(a\) and \(b\):**
   If at least one of \(a\) or \(b\) is \(< 1\), then it is possible to place the rectangle such that it does not cover any integer points. This is because the rectangle can be placed in such a way that it fits between the integer grid points without touching them.

4. **Sufficient Condition:**
   - If \(a < 1\) and \(b\) can be any value, the rectangle can be placed horizontally between two vertical lines of integer coordinates.
   - If \(b < 1\) and \(a\) can be any value, the rectangle can be placed vertically between two horizontal lines of integer coordinates.

5. **Necessary Condition:**
   - If both \(a \geq 1\) and \(b \geq 1\), the rectangle will always cover at least one integer point, either on its edge or inside it.

6. **Conclusion:**
   Therefore, the necessary and sufficient condition for placing the rectangle such that there are no integer coordinate points in its interior or on its edge is that at least one of \(a\) or \(b\) must be \(< 1\).

The final answer is: One of \( \boxed{ a } \) and \(b\) must be \(< 1\).