its in reference to the length of the midsection to accommodate the transom height. Normally its the measurement from the top of the clamp bracket to the mounting surface of the lower unit. That's kind of a hard place to measure, so most people just measure the transom from the top of the mounting surface to the keel, although its rarely exactly 20 or 25 inches. An easy way for me to identify length is looking at teh distance from teh lower motor mounts to the anti-ventilation plate. if its about a hands width, its a 25, if its only a couple inches, its a a 20. But that's not absolute. Originally, 15 inch was short, 20 was long, 25 was XL, 30 was XXL. Now we have 35 at XXXL, and I've heard of some boat manufactures making a 40( I haven't seen one) by taking a 35 and adding a 5 inch extension
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