Relief Area
The ‘relief area’ is the area in which you must drop your ball when taking relief.
In all circumstances, and irrespective of the club used to measure or to play the stroke, the relief area will be an area described by the longest club in your bag (other than your putter) using a reference point specified by the situation.
In general the size of the dropping area for relief incurring a penalty is based upon a measurement of two club-lengths and for free relief it is one club-length.
Back-on-line relief is always one club-length on either side of the line through the reference point to the flagstick, and as far behind the reference point as you wish.
More info here.
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