They measure from the base of the collar. But they also have a note that measurements have a tolerance of 0.5". I have found that their measurements are actually not consistent. I usually have to order multiple units of the same product/size and just keep the best example with least amount of tailoring needed.Not a direct answer... but, AFAIK, S&M does not post the method of how they measure anywhere on their website (which is pretty bad for a company doing so much custom). Personally, my default/standard method for measuring jacket length (for any kind of jacket) is down the back from the base of collar. But, I don't think that is what S&M uses. As you noted, I've also found that their sportcoats come in about 0.5" shorter than the measurement spec when using my measuring method... I have a theory that the measurement is from the top of the shoulder down...which will be longer from the middle back/base of collar. I do believe that their measurements are consistent tho... so I think it's safe to adjust your order accordingly. Would love to hear from others on this.