Among the Omahas the tipis were usually grouped according to gentes.[173] Tipi and wigwam are frequently used by us as synonymous, and in some dictionaries a picture of a tipi is made to represent a wigwam.[174] This is an error due to unfamiliarity with different forms of Amerind dwellings. The tipi is generally a portable structure while the wigwam is always fixed, and the latter is also of a different shape. Tipi is a Dakota term and wigwam is Algonquin. Tipi is really the plural for “house,” the singular being “ti,” and “pi” a termination indicating plurality.[175] It is constructed by arranging a number, sometimes as many as twenty or thirty, long poles, previously tied together near their tops, in a circle of about ten or fifteen feet diameter. This conical frame is then covered with bison hides sewed together in one sheet, or in modern days with canvas, shaped properly and laced or pinned together along the middle third of the junction of the covering mantle. The upper third is left loose, and its pointed ends are extended up and out by means of outside poles stuck into pockets in their extreme upper corners, according to the direction of the wind, to let the smoke escape from the fire built in the middle of the interior. If the wind blows straight at these flaps they are brought close together. Sometimes an extra skin is adjusted at the top so that it can be placed on any side to accomplish this object. The lower third is left open for a doorway, another skin being adjusted before it with a stick to spread it near its upper end, which end is attached to the tent. The bottom of the tent cover is held down by stakes or pins driven into the ground. In case of high winds, stones or other weights are placed on the bottom edge of the skins to keep them down. In summer the Omahas, and other tribes of the Dakotas, erected, when convenient, an elliptical lodge covered with bark, the roof being rounded and the construction being generally similar to the Algonquin elliptical wigwam. It was not more than seven feet high, while the tipi is twelve to twenty or more. These tribes also sometimes built earth lodges, chiefly for summer use, the roofs of which resembled in construction those of the Pueblo houses, though they were conical. A number of posts were set up in the ground to support in their crotches the transverse beams upon which numerous slender poles, about two inches in diameter, were laid to reach almost to the top where a hole for the exit of smoke was left. Against the outer series of posts all around slabs of wood were set up and the whole was then covered with earth a foot or two thick after matting and a layer of grass, or grass alone, was placed on the rafters or roof poles. This lodge was circular, the roof being conical, and it was entered through a covered way about ten feet long and five feet wide, the outer opening of which was protected by hanging bison hides. The supporting poles or posts