Each coracle is tailored to the local river conditions. In general there is one design per river, but this is not always the case. The Teifi coracle, for instance, is flat-bottomed, as it is designed to negotiate shallow rapids, common on the river in the summer, while the Carmarthen coracle is rounder and deeper, because it is used in tidal waters on the Tywi, where there are no rapids. Teifi coracles are made from locally harvested wood – willow for the laths (body of the boat), hazel for the weave ( in Welsh – the bit round the top) – while Tywi coracles have been made from sawn ash for a long time. The working boats tend to be made from fibreglass these days. Teifi coracles use no nails, relying on the interweaving of the laths for structural coherence, whilst the Carmarthen ones use copper nails and no interweaving.