This investigation examined the thinking of 128 pre-service teachers about the magnitude of large numbers and the relationships between large numbers. These pre-service teachers responded to a written task asking them to place one billion on a number line that included zero and one trillion and to provide an explanation for their placement. One billion was placed proportionately to zero and one trillion on the number line in only 20% of responses. The data revealed nine distinct strategies that emerged and different conceptions the pre-service teachers used for making the placements. The authors discuss possible reasons for the different conceptions.