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The kind of writing used changed too. In the earliest phase there were only the capital letters which got their refined and elegant forms in the classical periods in Greece (5th and 4th centuries BC) and in Rome (1st century BC). Minuscule writing systems (spreading letters between four imaginary lines, not two) replaced the majuscule systems in both Greek and Latin in the early middle ages. This again was an importan important bottle-neck for texts to pass: those who did not get transcribed into minuscule were nearly all lost.
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