Guides to arranging a shelf usually end with "less is more" and a vase. That leaves you in front of the empty board as before. Our rule can be measured: a third books, a third objects, a third air, on every level you look at. It is a rule of thumb from our workshop, not a standard, and it works because it governs two things at once, the eye and the weight. What it means in centimetres follows below.
23 centimetres per third
A KUUDU bay has 69.5 centimetres between the posts. A third of that is 23. On 23 centimetres stand twelve paperbacks, eight non-fiction books or seven academic books; the spine widths come from our weighing article, 1.9, 2.9 and 3.2 centimetres per volume. The books stand as a block against one post, upright and without a bookend, because the post holds them. The next 23 centimetres belong to one object, not three: a bowl, a picture, a plant, a single thing with space around it. And the last third stays free. Wall stays visible, and that is the point.
The second effect is the one you do not see: a third of books weighs just under four kilograms with paperbacks and just under seven with academic books. A board carries twenty. If you keep to the rule of thirds in the viewing zone, you automatically have light boards there and can put the weight lower down, where it belongs.
Only in the viewing zone
The rule applies to the levels you look at. Warehouse ergonomics divides a shelf into zones, and we translated them in the article on shelf heights: below 80 centimetres the stooping zone, up to 120 the reaching zone, up to 170 the viewing zone, above that the stretching zone. The rule of thirds belongs in the viewing zone, where it is decided whether a shelf looks calm or full. In the stooping zone the opposite applies: full boards, closed boxes, records, binders, the heavy things and the ones you rarely need. A shelf that is full at the bottom and a third free at the top looks like a shelf someone lives with.
The top level is set by body dimensions. According to DIN 33402-2 a small woman, fifth percentile, reaches 184 centimetres with both arms. Everything above 180 is stretching zone, storage for what comes down twice a year. If you apply the rule of thirds there, you put one object down and leave two thirds free; at the top a shelf may become empty.
| Zone | Height from floor | Arrangement | Books per third | Weight per third |
|---|---|---|---|---|
| Stretching zone | above 180 cm | one object, two thirds free | 0 | |
| Viewing zone | 120 to 170 cm | a third books, a third object, a third free | 12 paperbacks / 8 non-fiction / 7 academic | 3.7 / 4.3 / 6.7 kg |
| Reaching zone | 80 to 120 cm | two thirds books, a third free; the everyday | 24 / 16 / 14 | 7.3 / 8.7 / 13.3 kg |
| Stooping zone | below 80 cm | full: boxes, records, binders | 37 / 24 / 22 | 11 / 13 / 20 kg |
By subject, by author, by use
Within the books you need a second system, and each one costs something different. Ordering by subject matches how people search at home: you go to cooking, to architecture, to the novels. The price is the borderline cases, the cookbook with travel stories, and the question of how fine the subjects become. Ordering by author, alphabetically, finds fastest and tears subjects apart in return; it is the system libraries use within a subject, not for the whole wall. And ordering by use, what you are reading this season in the reaching zone, the rest above and below, is the most useful system and the only one that needs upkeep: twice a year, books move.
Colour is the special case. It looks good, nobody disputes that, and it works for people who remember books as images. Its price is double: finding takes longer when you do not have the cover in mind, and it ignores size. Heavy art books end up between paperbacks because they happen to be red, and that is the mix that conservators in the library article advise against: large volumes lie flat or stand with large ones, so the row stays straight. If you want to sort by colour, do it within a format: the paperbacks by colour, the art books next to them.