From the 2005 NEC. Note that there might be local amendments that changes this.
"(2) Wall Space As used in this section, a wall space shall include the following:
(1) Any space 600 mm (2 ft) or more in width (including space measured around
corners) and unbroken along the floor line by doorways, fireplaces, and similar
(2) The space occupied by fixed panels in exterior walls, excluding sliding panels
(3) The space afforded by fixed room dividers such as freestanding bar-type counters
This is from the Handbook. It is not code, but adds explinations and clarifications.
"A wall space is a wall unbroken along the floor line by doorways, fireplaces, archways,
and similar openings and may include two or more walls of a room (around corners),
as illustrated in Exhibit 210.24.
Fixed room dividers, such as bar-type counters and railings, are to be included in the 6-
ft measurement. Fixed panels in exterior walls are counted as regular wall space, and a
floor-type receptacle close to the wall can be used to meet the required spacing.
Isolated, individual wall spaces 2 ft or more in width are often used for small pieces of
furniture on which a lamp or an appliance may be placed, and to preclude the use of an
extension cord to supply equipment in such an isolated space, a receptacle outlet is
The word usable does not appear at all in 210.52 as a condition for determining
compliance with the receptacle-spacing requirements. As an example, to correctly
determine the dimension of the wall line in a room, the wall space behind the swing of
a door is included in the measurement. This does not mean that the receptacle outlet
has to be located in that space, only that the space has been included in the wall-line