Mixing geometric and floral motifs ought to be simple. A striped rug beneath a rose-print sofa, trellis wallpaper with botanical curtains, a chequerboard floor against a hand-painted leaf border. The contrast is the point: order against abundance, straight lines against soft ones.

Then the room is finished and something feels off. Most of the time the patterns themselves are perfectly good. The trouble lies in how they have been sized, coloured and repeated — and all three are easier to correct than they look.

Mistake one: picking patterns of a similar scale

This is the most frequent slip. A fine gingham check next to a small ditsy floral reads as one fuzzy texture from the doorway and as visual noise up close. The eye is searching for contrast; when both patterns sit at the same size, everything blurs into a single busy surface.

Separate them deliberately. If the floral is small and dense, the geometric should be large and open: wide awning stripes, a bold trellis, an oversized chequerboard. If the floral is blowsy — a big peony or poppy print — the geometric can afford to be finer: a slim pinstripe, a small regular dot, a narrow border.

Here is the test. Stand in the doorway, half close your eyes and look at the two patterns together. If they merge into a single grey buzz, they are too close in scale. Change one, usually the cheaper of the two.

Mistake two: making both patterns carry the colour

Two multicoloured patterns in one room will argue, particularly when both are saturated. A floral in hot pink, lime and orange beside a geometric in bold primary colours gives the eye nowhere to rest, and the room feels smaller than it is.

Let one pattern take the colour job and keep the other tonal. A floral in greens and soft pinks sits happily with a geometric in two shades of a colour already present in the floral — a pale grey-blue stripe with a deeper blue line, for example. The geometric supports the scheme rather than competing with it. Restricting both patterns to three colours in total, including their backgrounds, is a reliable way to keep the peace.

Check the background, not just the motif

Base colours matter as much as the patterns themselves. A floral printed on cream placed beside a geometric on brilliant white looks unresolved even when the two motifs are well matched. Put the samples side by side and compare the grounds first. If one is warm and the other cool, neither pattern will ever settle.

Mistake three: repeating the motif until it becomes a uniform

When a pattern works, the temptation is to use it on the curtains, the cushions, the lampshade and the headboard. At that point it stops being a motif and becomes a uniform, and the room loses depth.

Repeat the idea, not the fabric. If the geometric appears as a striped rug, echo it with a slim stripe on a cushion edge, a length of striped trim along a blind, or a single painted line on a piece of joinery. The eye makes the connection without being shouted at.

Mistake four: leaving the two patterns touching

Every pattern needs a plain neighbour. Where a geometric and a floral meet directly — a patterned cushion on a patterned sofa, a tiled splashback directly above a botanical wallpaper — the edges do battle, especially when the two sit at similar tones.

Insert a pause. Plain piping on a cushion, a solid-colour seat pad, a plain painted dado rail, a rug with a wide unhurried border. A folded throw in one flat colour over the arm of a patterned chair costs very little and settles an entire corner.

Two patterns, then a plain. That rhythm solves more decorating arguments than any amount of careful colour matching.

Mistake five: putting pattern on the wrong surfaces

Think about which surfaces will change and which will not. Tiles, wallpaper, fitted joinery and flooring stay for years; curtains, cushions, lampshades and throws can be swapped in an afternoon.

If you are drawn to a bold geometric, give it the fixed surface and keep the floral on soft furnishings — or the reverse, as long as the pattern you might tire of is the one you can change. Getting this back to front is what leaves people feeling stuck with a room they no longer like.

Room size matters too. A large open geometric on the floor of a small room compresses it; the same pattern in a generous hallway gives the space a stride. Choose scale to suit the room, not your enthusiasm for the design.

Mistake six: judging the mix flat, in daylight, on a table

Two swatches lying side by side on a kitchen table tell you very little. Fabric behaves differently when it is gathered into a curtain, stretched over a sofa or lit by a lamp at nine in the evening.

Pin the samples where they will actually live. Drape the floral over the arm of the sofa, tape the geometric to the wall or lay it on the floor, then look at them in the morning, in the afternoon and under lamplight. A pairing that only works in bright daylight does not work.

Fixing a mix that has already gone wrong

You rarely need to start again. Work through these in order and stop as soon as the room settles:

1. Remove one pattern. Take away the cushions first, then the throw. Often the room improves straight away.
2. Add a plain. A solid-colour rug, cushion or lampshade gives the remaining pattern room to breathe.
3. Change the scale of the smallest element. New covers on two cushions in a much larger or much smaller print is the cheapest correction available.
4. Pull one colour across. Introduce a single shade from the floral into the geometric, or the other way round, in a small quantity: a trim, a frame, a vase.
5. Bridge the grounds. If both patterns sit on different whites or neutrals, place something plain between them — natural linen, a warm oatmeal wool, a painted panel — to link the two.

Keep the whole room in view. In most successful schemes the plains take the largest share of the space, one motif leads and the other answers it. If you can name the leading pattern from the doorway, the balance is probably right.