By 1800 guitars with six single strings (tuned EAdgbe’) had become the norm. The rosette gave way to an open sound hole, while the neck was lengthened and fitted with a raised fingerboard extending to the sound hole. Nineteen fixed metal frets eventually became standard, the top note sounding b”. The bridge was raised, the body enlarged, and fan-strutting introduced beneath the table to support higher tension strings. Treble strings were made of gut (superseded by more durable nylon after World War 2), bass strings from metal wound on silk (or, more recently, nylon floss). Tablature became obsolete, guitar music being universally written in the treble clef, sounding an octave lower than written. By the 1820s makers such as Louis Panormo of London were replacing wooden tuning pegs with machine heads for more precise tuning, and creating the prototype of the modem classical guitar (a design perfected in mid-century by Antonio Torres).