If you own an older home, you may find yourself with two different profiles of baseboards. The two profiles can be difficult to flow seamlessly. You can do a few things to make a transition from one profile of molding to another so the molding differences are unnoticeable.
Baseboard moldings come in a large variety of styles and profiles. Some are flat, others highly detailed. The easiest way to make a transition from one type of molding to another is to try to match the moldings as closely as possible. If the existing molding is flat, look for another type of molding with a similar profile. Even if you can't get the exact same profile, the slight differences aren't perceptible to most people.
Baseboard molding transitions work best when both baseboards are the same height and width. Measure the existing baseboard molding in your home and use the measurements to buy the new molding. If you find a profile that you like that isn't tall enough, install the molding to the same height as the older baseboard molding. Cover the gap between the floor and the molding with quarter round molding. Maintain the look of the molding by installing quarter round in front of the older molding, too.
The easiest way to make a transition from one molding to another is at a corner. There is a natural break at a corner that makes it easy to end one type of baseboard molding and start another. Whenever possible, keep the molding consistent in one room and make a transition in a hallway. Transitions can be done in the middle of the wall, but it generally looks better if you tear out old molding and make the whole wall one type of baseboard molding.
Putty and Caulk
Wood putty and caulk are the DIY molding installer's best friends. This is because you can fill in large gaps and cracks between molding pieces with putty or caulk. This also means that your transition between two different types of molding doesn't have to look bad. Fill the gap between the moldings with the putty and let it dry. Once the putty is dry, lightly sand with sandpaper and then paint or stain the moldings as desired.