So horizontal lines are drawn in front of the plane coming out of the board, while vertical lines are drawn in the back of the plane going into the board. So a fissure projection is nothing more than a way to basically describe the three dimensional stereo chemistry of the sugar molecule. Now, the final thing I'd like to discuss are diary, isomers and ephemeris. And as we'll see in just a moment, an epimer is basically a specific type of diaryoisomer. So let's think back to organic chemistry for just a moment. So what exactly is an isomer?