A thumbnail is a composition tested while it is still cheap. In a box the size of a large postage stamp you can move a horizon, enlarge a figure or darken a corner in one minute. The same experiment on a stretched canvas costs an afternoon and a quantity of pride. The habit is to make several of these boxes before the real surface gets a mark.

Same proportion, almost no detail

Draw the boxes in the same proportion as the painting you think you are about to make. A square thumbnail will lie to you about a long landscape. Inside each box, forbid detail. No bricks, no eyelashes, no lettering. Flat shapes only: the light mass, the dark mass, the thing that should lead.

Value First: Why a Grey Study Saves the Painting is the close cousin. A thumbnail can be that grey study and a composition study at once, if you let it stay small. The moment you render a thumbnail, you have started the painting in the wrong place and you will be reluctant to throw it away.

Change one decision per box

Give each box a different job. One puts the subject on a third. One centres it. One crops close. One makes the sky the largest shape. The Rule of Thirds, and When to Break It is a prompt, not a quota. You are looking for the arrangement that reads when you lean back, not for the arrangement that obeys a rule.

Change the value pattern as well as the placement. A light object on a dark ground in one box, the reverse in the next. If every thumbnail has the same contrast, you have not tested anything. You have repeated a doodle.

How to choose without bargaining

Prop the page up and look at it from a few steps away, or hold it at arm's length and squint. The box that still explains itself is the candidate. If two seem equal, they are probably both acceptable, and fussing between them is delay. Pick one with a mark in the corner and write a single sentence: dark trees against a light wall, figure low and to the left. That sentence is the brief for the painting.

Do not start the large work from a new impulse that arrived while you were squeezing paint. The thumbnail was the thinking. Scale: Letting One Element Clearly Lead should already be visible in the box you chose. If nothing leads at that size, it will not lead when it is larger. Draw another box.

A row you can keep

Work the thumbnails in the sketchbook, not on scrap that you bin. A row of rejected boxes is a record of your taste improving. Later you will see that you always centre the subject, or always leave a blank band at the bottom. That repetition is the lesson. Blank Page Fear: Gentle Ways to Start a Sketchbook Page gets easier when the first marks are allowed to be this small and this disposable.

Five boxes are enough. Ten become a way to avoid painting. When the fifth is done, choose, and go to the surface you actually meant to use.

Frequently asked questions

How small is small enough? Small enough that a face is an oval and a window is a block. If you can draw a catchlight, the box is too big for this exercise.

Should thumbnails be in colour? Value first. A second row in three colours is useful once the design holds. Colour on an unresolved box only makes a weak plan look festive.

Can I use thumbnails for a layout? Yes. The same boxes will test a poster. One large word, one image, different crops. The discipline does not care whether the final surface is canvas or paper.

Steps

Draw several small frames: Box off four or five rectangles in the same proportion as the painting you intend.
Block values, not details: Fill each box with a few flat tones and move the main mass to a different place in each.
Choose one and stop: Pick the thumbnail that reads at arm's length and begin the painting from that plan, not from a new idea.