Different Piece of Carpet
If you have a large gap, give a room a decorative accent by filling the gap between two carpet pieces with a decorative carpet strip. Measure the gap between the two carpets, and cut the decorative piece of carpet 1/4 inch wider than the measurement. Nail down carpet tack strips along both of the edges of the gap directly beside the two carpet pieces. Place the decorative piece of carpet into the gap,and hammer the carpet into place using a rubber mallet. When the tacks catch on the carpet, it holds the decorative carpet in place, filling the gap.
Metal Threshold Plates
Metal threshold plates are the easiest method for filling small gaps between different types of carpet. These plates come in sizes from 1-inch wide to 8 inches wide and come in a variety of different colors. Measure the gap between the two types of carpet, and get a threshold plate 2 inches wider than the gap. Place the metal plate over the gap so that it is 1 inch over each piece of carpet. Nail or screw the threshold plate in place. Each kit comes with the required nails or screws.
Peel-and-Stick Tile Transition
Another way of filling a large gap is filling the area with tile. Use peel-and-stick tile for filling the opening between the two types of carpet. Measure the pieces, cut them to fit, peel the backing and stick them in place. The tiles come in many different colors and patterns so there is one that fits your needs.
Wooden Threshold
Wooden threshold plates are just like the metal threshold plates. They come in different sizes, ranging from 1 inch to 8 inches wide, so they work well for small gaps. They come in a wide variety of colors. If you want a larger area covered, make your own wooden threshold. Measure the opening, and cut a 1-inch thick board so it fits the opening. Use a router on the edges of the board so the sides are rounded, and nail it in place.