A French door is really a pair of doors pretending to be one: two full-glass leaves hung on opposite jambs, meeting in the middle. The style is centuries old because the idea never stopped working, and it remains the doorway that makes a dining room, a patio wall, or a grand entry feel like an occasion.
How a Pair Actually Works
One leaf is the active door, the one with the handle you use daily. The other is the passive leaf, held closed at the top and bottom by bolts until you release it, and then the entire opening is clear. Between them runs the astragal, a weatherstripped spine fixed to one leaf that the other closes against. That strip is the whole game in a French pair: it is what separates a sealed double doorway from two doors with a draft down the middle.
Locking follows the same logic as a good entry door. The active leaf throws a multipoint lock into the passive leaf, and the passive leaf bolts into the frame above and the sill below, so the pair braces itself rather than rattling against itself.
Where the Width Pays Off
The obvious answer is furniture: a sofa, a piano, or a grill goes through an open pair without the tilt-and-pray routine a single door demands. The daily answer is lighter. Opened wide on a summer evening, a French pair turns a dining room and a patio into one room, and even closed, two leaves of glass put more daylight into the back of a house than any window the same wall could carry.
French Pair or Hinged Patio Door?
A hinged patio door is one swinging leaf beside a fixed glass panel; a French pair swings on both sides. Choose the pair when the opening is genuinely wide and you want all of it, and the single hinged door when the wall is tighter or the budget wants the fixed panel. Either way the leaf seals by compression against the frame, the same way a front door does.
Living With a Pair
Use one leaf daily and let the passive leaf stay bolted until you need the width; the astragal and weatherstripping last longer that way. The hinges of both leaves appreciate the same drop of oil an entry door gets, and blinds sealed between the glass panes keep the privacy question answered without anything to dust.