Some critics express the opinion that it is only from the mid-19th century, and especially in the 20th century, that the score began to hold such a high significance. Previously, improvisation (in preludes, cadenzas and ornaments), rhythmic flexibility (e.g., tempo rubato), improvisatory deviation from the score and oral tradition of playing was integral to the style. Yet in the 20th century, this oral tradition and passing on of stylistic features within classical music disappeared. Instead, musicians tend to use just the score to play music. Yet, even with the score providing the key elements of the music, there is considerable controversy about how to perform the works. Some of this controversy relates to the fact that this score-centric approach has led to performances that emphasize metrically strict block-rhythms (just as the music is notated in the score).