Inseam is the distance from your crotch to the ground.
You now know approximately the suitable frame height (seat tube length) for your new bike.
What frame size your bike should be is also influenced by other factors:
Arm length and shoulder width : These measurements also play a role and should be taken into account when buying a new bike to determine frame size and component selection as accurately as possible.
Intended use: Are you going to cycle sportily or recreationally? When in doubt, choose the smaller frame for sporty riding and the larger one for long leisurely rides.
Desired sitting position : Unlike the upright seating position on a touring bike, the seating position on a road bike is angled forward, which reduces drag, among other things. Sitting position and frame size are also related.
If in doubt contact a Thompson dealer to determine the correct size.
Inseam is the distance from your crotch to the ground.