Exodus 30:2 A cubit shall be the length thereof, and a cubit the breadth thereof; foursquare shall it be: and two cubits shall be the height thereof: the horns thereof shall be of the same. - Free Bible Online

      2-4. foursquare--the meaning of which is not that it was to be entirely of a cubical form, but that upon its upper and under surface, it showed four equal sides. It was twice as high as it was broad, being twenty-one inches broad and three feet six inches high. It had "horns"; its top or flat surface was surmounted by an ornamental ledge or rim, called a crown, and it was furnished at the sides with rings for carriage. Its only accompanying piece of furniture was a golden censer or pan, in which the incense was set fire to upon the altar. Hence it was called the altar of incense, or the "golden altar" [Ex 39:38; 40:26], from the profuse degree in which it was gilded or overlaid with the precious metal. This splendor was adapted to the early age of the church, but in later times, when the worship was to be more spiritual, the altar of incense is prophetically described as not of gold but of wood, and double the size of that in the tabernacle, because the church should be vastly extended (Mal 1:11).

JFB.