The dropout is the space on the frame where your rear axle slides in when attaching the rear wheel to the bike. My bike has a mostly horizontal drop out, much like BMX bike, which gives room to pull the tire straight back to tighten the chain. Most newer mountain bikes and multispeed bikes have vertical dropouts that the wheel slides straight up into, rather than straight back. If yours is vertical, you will have really no way of tightening the chain on the engine side. However, some people get lucky with taking out just the right amount of links lets them have the perfect chain tension.