Just about the time the leaves begin to show their little green nubbins, the cook's mind turns inexorably to thoughts of strawberry tarts. (Some cooks' minds, anyway; others' minds turn to thoughts of asparagus and leafy green vegetables, but that's another story.) The strawberry tart season is fleeting -- though not as fleeting as it once was -- but fortunately it's followed by the peach, apricot and blueberry tart season, then the raspberry tart season, and finally, to take care of the rest of the year, the apple and pear tart season.

What distinguishes a tart from a pie? Mostly the form the thing is baked in. Historically speaking, Americans make pies instead of tarts, and the pies are baked in slant-sided pie tins to be served directly from the tin. Europeans make tarts, which are baked in straight-sided tins and usually served free-standing.

If you want to make fruit tarts, you need to think in terms of tart rings or tins (sometimes called "flan" rings or tins, flan being another word for tart). Tart rings are just that -- metal rings that are used in conjunction with baking sheets to form and bake the tart shell. You simply plop the ring down on a baking sheet and line it with pastry dough as you would a regular pie tin. When the shell is baked you slip the ring upward (the pastry shrinks just enough to let you do this easily), slide the baked shell off the baking sheet and voila , you're ready to fill it.

The tart tin is based on the same idea, but it includes its own separate, perfectly flat bottom that fits down inside the ring. (The ring has an interior lip just wide enough to hold it.) When it's time to unmold the tart shell, you set the tart on a wide jar or can, and the ring slips down. To serve the tart you can leave it on the metal base for extra fortification, or slip it off and serve it on its own.

Because of the nature of their design, both rings and tins tend toward a certain flimsiness, so it's important to buy the sturdiest ones to begin with. Some flan rings come with rolled, and therefore reinforced, edges, which make them much less prone to bending and warping.

It's equally important to use a heavy-gauge baking sheet with tart rings. A flimsy baking sheet that bends double when it's put into a hot oven will produce a funny-looking tart shell. And it's not a bad idea to reinforce a tin by using a baking sheet underneath it.

Tin-coated steel is the usual material for these items, but they also come in black or "blued" steel. The dark steel seems to produce a darker, sturdier crust faster, which can be a virtue in tart shells that have to hold fruit and pastry cream on their own. These dark steel tins tend to be very sturdy. Any steel pan needs to be dried thoroughly after it's been washed and should only be washed when absolutely necessary.

Ceramic quiche pans are lovely, but they are more suited to savory tarts that need longer, slower baking. If you want to bake a tart shell in them, let a sturdy cookie sheet preheat, then lay the pan on it for a quick burst of heat that will help to dry out and brown the crust.

The standard tin is a simple round shape, but nothing is ever simple for long, and pretty soon the eye travels to the flower shape, or the boat shape, or the stately rectangle.

Since tarts are usually served free-standing, the sides need to be sturdy enough to stand on their own, even after the tart is filled. Take care of this as you fill the tin with rolled-out dough, by pressing extra dough down into the sides to make them slightly thicker than the bottom.

In some ways the tiny tins -- in rounds, rectangles or boat shapes -- make the most beautiful fruit tarts. The smaller shapes can be bought in individual pieces or stamped in sixes or twelves into one large sheet. The ones stamped into one large sheet are somewhat easier to fit the dough into and to bake. The tiniest rounds are perfect for one strawberry; the tiny boat shapes (usually called "barquettes") are nice for an artfully arranged row of strawberry slices.

Since shells for fresh fruit tarts are usually baked "blind," or without any filling, you have to keep the dough from puffing out of shape as it bakes. You can prick the shells with a fork, line them with foil and fill them with dried beans, rice or the little metal nuggets sold just for the purpose. But this can get tiresome when you are doing three dozen little tartlets. Some bakers buy twice as many molds as they need and use the extras as anchors, setting them down on top of the pastry as it bakes; or you can simply prick the dough and chill it thoroughly before baking it.