SOS:
The answer is CLARITY!
The ABC's of good letter writing apply more particularly to business letters than to any other letter; however, in many cases, they are true of all letter writing. A stands for accuracy, B for brevity, and C for clarity. Accuracy basically means authenticity, honesty, and faithfulness to the facts of the case. Brevity means conciseness. Clarity means intelligibility, comprehensibility, explicitness, precision, and plain speaking.
Hope this helps!!