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Linear feet just means any distance or length measured in feet. The term in home repair is usually applied to products that are installed without regard to other dimensions, such as width or thickness.
Because there are other types of "feet", square feet and cubic feet, product manufacturers had to come up with a distinguishing name for plain old length… hence the term "linear" feet.
For example, if you were going to purchase ceramic tile, you would normally order the tile by the square foot… logical since that is the most reasonable way to measure the floor… LENGTH (in feet) multiplied by WIDTH (in feet) equals SQUARE FEET. However, if you were to purchase a tile bullnose (covers outside corners) or a decorative tile border, it would be ordered in linear feet. You would measure the LENGTH only of the area you want to cover… the width is irrelevant (except for decorating and aesthetic purposes, of course!).