Most patterns are cut too low under the arm. Deciding where the armhole hits under the arm is partially personal preference. An armhole cut high up under the arm is generally more comfortable because it allows a greater range of movement. A sleeveless garment is only 1/2" higher under the arm than a fitted garment with a sleeve. Hold a ruler under your arm as high as comfortably possible, the underarm seamline should fall barely below where the ruler is touching the flesh.