The number of single churches scattered over the islands and the adjoining mainland has at one time been very large. Many of these are now reduced to mere traces, but the ruins of a considerable number still exist in tolerable preservation. We shall consider these in several sections, in accordance with their different characteristics. The simplest type is first described, but the age of the structures is not thereby indicated. 1. Churches built in the simple Celtic style of one oblong chamber. A few of these structures are evidently very ancient, the walls having been built without cement, or having doors with inclined jambs. These will be first described. 2. They will be followed by a long list of churches on the same general plan, but in which some modifications of the original arrangement of one door and one window are introduced. The number of windows is often increased; but they are not arranged on a uniform plan, being generally distributed so as to suit the locality. The windows were no doubt open and unglazed, and seem to have been placed so as to be well sheltered, and as little exposed to the prevailing wind as possible. In some cases there is no window even in the east gable, and the west and north walls are frequently blank. In most examples the windows and doors are narrow and flat-headed, but round-headed doors and windows are also occasionally used, showing the Norman influence. These structures all belong more or less to the primitive type of Celtic churches, but their primitive features cannot be relied on as fixing their date. Some may be very early, while others may be later even than the pointed buildings to be afterwards described. The primitive type and primitive features, such as narrow flat-headed windows and doors, undoubtedly continued in use long after more advanced forms had been introduced. 3. A more reliable mark of the order of succession of these early churches is the existence of an architecturally distinguished chancel. At what point in the development of the architecture this feature was introduced it is impossible to say; but from the extremely rude nature of the other elements in such examples as that at Lybster in Caithness (described below), it must clearly have been at an early period. We there find the plan of nave and chancel combined with such primitive features as a low west doorway, with inclining jambs and flat lintel, and the opening to the chancel from the nave shaped exactly like the west doorway. 4. Examples are also found of the addition of a chancel or nave to an older simple oblong church, thus showing that the chanceled church was a later form of plan than the primitive single-chambered church. The examples of these ancient churches with added chancel or nave will be first given, and then the churches built on the plan of nave and chancel.[79]