Sounds, too often repeated, no longer impress themselves on consciousness. The counting of imaginary sheep... one after the other... is a remedy for insomnia. And monotonous sentences, sufficiently repeated, can wear down the strongest resolution to study, the best intentions to learn. We react to monotonous repetition by simply withdrawing attention. Sentences may have impeccable clarity, complete objectivity, exquisite precision, and yet completely fail to hold the attention.
Writing aims to communicate, but if it does not hold the attention, it fails of its purpose and might as well not have been written. Unfortunately, monotony often exerts its deadly effect without either the author or the reader having any awareness of the fault. Yet the fault is quite readily corrected, if only we are aware of it.
Monotony can take several forms. I would consider, primarily, monotony in regard to sentence length and sentence structure. While the length of the