Idiomatic Phrases

Look at the following two sentences and decide which is idiomatic:

1. Angela speaks at length about the company's new payroll procedures.
2. Angela speaks lengthily about the company's new payroll procedures.

The first sentence is the correct idiom. Why is it correct to say "at length" but incorrect to say "speaks lengthily"? There is no specific reason. "At length" is simply a common phrase that developed over time, and has come to be accepted as linguistically correct.

Use your ear to test out idioms, and memorize any unfamiliar idioms that you encounter before test day.