Here a convex vinyl ridge across the top of the threshold presses against the bottom of the door for a tight seal against drafts. Another type, less often used and recommended in cold climates, is the interlocking threshold shown in. Although very effective when in good condition, the interlocking elements are difficult to install and easily damaged. Special tools are required, and complicated adjustments are frequently necessary. You'll find a half threshold necessary where two floors of different height come together.
Half saddles are available in metal or can be made by adapting a wooden saddle.
You will need to know the difference in floor heights before purchasing this threshold. Paint stiffens the vinyl and diminishes its sealing ability. These products are used to close the gaps between the threshold and the bottom of the door. Usually, removal of the door is not required for door sweeps. You'll find these to be easiest to apply and inexpensive, thus the most popular.