Alignments or stone rows are groups of standing stones set in straight line. They can occur in any size group from two (usually refered to as a stone pair - see Boherboy (County Dublin)) to ten or more, although anything over four is exceptional (see Castlelanaght (County Cork)).
The function of these is a bit of a mystery, although many do seem to have significant astrological or geographical properties.
The stones making stone pairs often appear to be totally different in shape from each other, often hinting at a male and female partnership.