The consonants were already there, and the Masoretes did not alter them. What they supplied was everything around the consonants: a system of points and strokes recording the vowels, a second set of marks recording the chant and with it the phrasing and the syntactic division of every verse, and marginal and end notes counting words and letters, flagging unusual forms and recording how often a spelling occurs. The notes are a checking apparatus, designed so that a copyist's error becomes visible.
One device shows the method most clearly. Where the transmitted consonants were held to be wrong or unsuitable, the Masoretes did not correct them. They left the written form untouched and noted in the margin what should be read instead, preserving both the received text and the required reading without letting either displace the other. More than one system of vocalisation was developed, in different centres and on different principles, some placing the marks above the line rather than below, and the Tiberian system eventually displaced the others so completely that the alternatives are now known mainly from fragments recovered from a synagogue storeroom. What is read today as the pointed Hebrew Bible is one particular school's work, produced by a small number of families in one town over a few generations, and it prevailed over its rivals rather than being the only thing there was.