In electoral systems with districts that vary in magnitude, the number of seats to be filled in each district will be even or odd. We argue that such a variation has crucial political consequences, called the ‘odd–even effect’. In low-magnitude districts, elections are more competitive when the district magnitude is odd than even; the incentives for coordination are thus stronger in the former scenario than in the latter. Employing quasi-experimental data from 780 districts in Spain's lower house elections, we show that the number of parties is smaller in low-magnitude districts with an odd number of seats than in low-magnitude districts with an even number of seats. The elite- and voter-level mechanisms driving the odd–even effect are examined using data on mobilisation efforts and wasted votes at the district level.