And so if this is our test tube, this way it's towards the bottom of that test tube and this way it's towards the top of that test tube. And so if the band appears lower that means the DNA molecule that is described by that band has a higher density because the farther we go to the right along the x axis, the more dense our molecule is. And so initially what our initial sample showed was we had a single band. Now this single band basically described a single type of DNA molecule in which both of those strands consisted of that isotopic N 15 atom. So this describes the N 15 DNA molecule and so this is what we see. So one of these strands consists of the N 15 and the other strand also consists of the N 15.