"Extended Precision" is a part of the specification. The "extended precision" requiremnent for a given type requires that the "extended precision" type be AT LEAST as precise as the base type, up to the precision of the next larger type. base-2 floating point typically has "standard" types for 32-bit, 64-bit, 128-bit, 256-bit, 512-bit, and 1024-bit representations. The "extended" 64-bit format must have at least the same precision as the standard 64-bit float and not more than the precision of a 128-bit float, however, you are correct that the representation of a base-2 extended precision floating point number is implementation defined. Typically, a "long double" is ten bytes (80-bits) simply because the Intel 8087 math co-processor used that format internally -- it is how the IEEE requirement to "compute exactly then round half-even" was implemented (which implementation was really "compute with guard bits then round"). Some compilers treat "long double" as a 96-bit "storage space" (12 bytes) but what is stored there can be anything ranging from a IEEE 64-bit double up to some extended precision encoding. Some compilers treat "long double" as a 128-bit space (one paragraph or 16 bytes) and may store in that space anything from a single IEEE 64-bit float (with padding), some internal representation that is implementation specific but more precise than a IEEE 64-bit float and less precise than a IEEE 128-bit float, or maybe even a IEEE 128-bit float.