What it is
Crème légère is creme patissiere with whipped cream folded into it. Nothing else is added and nothing else sets it: the dataset records its setting agent as none, which is the whole character of the thing. The starch that set the base is still the only structure in it, now diluted by the cream that was folded in.
So it is lighter and softer than the base and, unavoidably, less stable: whipped cream folded into loosened cold pastry cream. no gelatin, so it will not hold a tall structure for long. In practice that makes it a filling to assemble and serve, not one to build with and leave standing.
Against crème diplomate, which is the confusion worth clearing
Both are pastry cream lightened with whipped cream. Read most recipes and the two look interchangeable. They are not, and the difference is a single step that happens before the cream is folded in: for a diplomate the pastry cream is set with gelatin first, then loosened, then lightened. The pastry cream is set with gelatin FIRST, then loosened, then lightened with whipped cream. The gelatin is what separates it from creme legere.
That is why a diplomate is the cream in a fraisier and a légère is not. A cake that has to hold a vertical wall of cream between two layers of sponge needs the structural version; airy and light, but structurally stable against lighter, softer, less stable. Choosing a légère for that job produces a cake that looks correct for an hour.
The full family (six members, one base, one distinguishing addition each) is tabulated on the crème pâtissière page. The reason the gelatin matters at all, rather than being a detail of one recipe, is in setting agents.
When it is the right choice
When the assembly is shallow and immediate. A fruit tart filled and served the same day is the classic use, and the lightness is a genuine gain there: the base alone can taste heavy under fruit, and thinning it with cream rather than with milk keeps the flavour and loses the density.
When it is the wrong choice: anything piped tall, anything cut into slices that have to stand, anything made the day before. For those the answer is a diplomate, or crème mousseline if a richer cream suits. That one is lightened with butter (beurre pommade) instead, and none - the butter sets on chilling, which is what holds a tarte tropézienne together.
Questions about crème légère
What is crème légère?
Crème pâtissière with whipped cream folded into it, and nothing else. No gelatin, so the starch in the base is the only structure and the result is softer and less stable than the base was.
What is the difference between crème légère and crème diplomate?
Gelatin. A diplomate sets the pastry cream with gelatin before loosening and lightening it, so it holds a tall structure. A légère skips that step and will not.
Can I use crème légère in a fraisier?
It is the wrong cream for it. A fraisier needs the cream to hold a vertical wall between sponge layers, which is what the gelatin in a diplomate is for.
How long does crème légère keep?
It is a same-day filling. Nothing in it sets beyond the starch, so it slackens as it stands and the whipped cream weeps in time.
Is crème légère the same as crème chantilly?
No. Chantilly is sweetened whipped cream on its own. Crème légère is pastry cream with chantilly folded into it, so it has the starch, the eggs and the flavour of the base in it.