This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Often strings, more especially in Gothic buildings, by following the curves of the arches, either of windows and doors if external or of the nave arches internally, serve to accentuate the arch mouldings and also externally to protect the tracery from the weather, etc. The more appropriate term for those occurring on the inside of structures is hood-moulds. Fig. 147 illustrates a few forms of Gothic string courses, such as might be used either horizontally or as hoods. When horizontally applied in Gothic structures they particularly serve to unite the buttresses, piers, and other parts of a building, by being carried round them, also frequently forming a basis on which the sills of the window rest.
In shafting, frequent use is made of strings or bands, so as to break the uniformity of clusters of columns of great height, forming a rest for the eye and giving an appearance of tying in and strengthening the groups.