Q:Which types of boards have four holes where the binding threads were laced through?  Answer the above question based on the context below:  The boards of the covers, previously assumed to be limewood, are now thought to be birch, an unusual wood in later bindings, but one easily available in northern England.  Both have four holes where the binding threads were laced through; the two threads were run round the inner edges of the cover and knotted back at the holes.  The front cover has an additional 12 holes where the ends of the cords for the raised framing lines went through, at the four corners of the two main frames, and the ends of the horizontal bars between the interlace panels and the central vine motif.The stitching of the binding uses "Coptic sewing", that is "flexible unsupported sewing (produced by two needles and thread looping round one another in a figure-of-eight sewing pattern)"  Coptic sewing uses small threads both to attach the leaves together and, knotted together, to attach the pages to the cover boards. Normal Western binding uses thread for the former and thicker cord running across the spine of the book for the latter, with the thread knotted onto the cords.  Coptic sewing is also found in the earliest surviving leather bookbindings, which are from Coptic libraries in Egypt from the 7th and 8th centuries; in particular the design of the cover of one in the Morgan Library (MS M.569) has been compared to the St Cuthbert Gospel.  In the techniques used in the binding, apart from the raised decoration, the closest resemblance is to an even smaller Irish pocket gospel book from some 50 years later, the Cadmug Gospels at Fulda, which is believed to have belonged to Saint Boniface.  This is also in red goatskin, with coloured incised lines, and uses a similar unsupported or cordless stitching technique.  The first appearance of the cords or supports that these "unsupported" bindings lack is found in two other books at Fulda, and they soon became universal in, and characteristic of, Western bookbinding until the arrival of modern machine techniques.  The cords run horizontally across the spine, and are thicker than the threads...
A:
limewood