A well-tailored suit is the mainstay of men's fashion. To achieve a truly perfect fit, the tailor fits the garment to all the measurements of the wearer's body. These measurements include the shoulders, the length and width of the arms and legs, the waist, the collar and the chest. To take the measurement of the chest, tailors measure from armpit to armpit. If you need to take your own measurements, you can do the same easily.
Place one end of the tape measure at the edge of your left armpit. Don't actually tuck the end under your arm.
Hold the tape measure in place with your left hand and draw it across your chest to your right armpit. Don't actually bring the tape measure under the arm. The line of the tape measure should run across the center of your chest, along the line of the nipples.
Mark the place where the tape measure reaches the edge of your armpit with a finger.