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The cell membrane is a double-walled structure containing a cell's genetic code.
A cell that is double walled is a plant cell. This is because the plant cells can't move so they need extra protection.
Luxury caskets often have a double lid, sometimes even a triple lid (the innermost being made of glass). In rare cases, caskets are even double walled (triple walled would be absolutely unusual), which means that the outer casket contains a complete inner casket, which can be taken out. Double walled caskets are much more heavy than double lid caskets, of course.
Pericardium
Serous membrane
A double phospholipid bilayer surrounds the nucleus.
Phospholipid Bilayer
Amniotic sac
mitochondrion
Ice is readily available for them, and it is a good insulator.
Such organelles are called mitochondria.
Another muddy question. Rims on a bike can be either single-walled or double-walled, with double-walls being the stronger choice. Never heard of triple walled. Cranks OTOH can have either a single chainwheel, double chainwheels or triple chainwheels. Singles are most common for bikes that are single-speed, have internal gears, or sports bikes built for a specific purpose, like BMX, dirt or track bikes. Double chainwheels are pretty much the standard for road bikes, while MTBs, hybrids and touring bikes often have triples to widen the range of available gears.