Ok this is embarrassing but I'm going through "drawing on the right side of the brain" and I'm at the part now where it teaches me about negative space and the "basic unit" (first shape you put down) and that the basic unit's proportions determine all other proportions in the drawing. so far so good, but then it says that I should put my pencil on Vincent's chair to compare the relationships, like to compare the width of all 3 negative spaces in the chair back.
and it says that for each proportion I'm supposed to go back to my basic unit to measure it and make the comparison with the other negative spaces in the chair back. and then it says that these relationships are expressed in relationships to "one" or in other words "ratios"

then it says, the negative space just below our basic unit is about 1:7/8, but I can't see it, and I also don't know if the "1:7/8" stands for "width:height" or "basic unit width:other negative space's width" or what. Like is the "one" the space I'm currently looking at, or is it the basic unit, and is it the vertical or horizontal line or the whole thing? I'm utterly confused. It also says the next space below is 1:3/4.
i have no idea how important it is to grasp this.