Standing stones, also called menhirs or monoliths, are the most simple of megalithic monuments. They are exactly what they say, a stone that stands with one end set into the ground. Being simple in form does not make them simple to understand, for they have served several purposes over time. Some were placed to mark burials, others were probably erected to mark boundaries or travel routes, the purpose of others is uncertain, but it may well have been ritual.
Standing stones can vary enormously in size from a under 1m tall to over 4m. Some have been purposely shaped (see Stone Of Destiny (County Meath)) and some must have been chosen purely for their shape (see Ballyvatheen (County Kilkenny)). Most standing stones are dated to be from the Bronze Age, but some are clearly older, especially those associated with passage tombs such as at Knowth (County Meath) and Loughcrew - Corstown (County Meath).
Others have been re-used in later times (see Kilnasaggart (County Armagh) and Breastagh (County Mayo)), perhaps to try and capture some of the powers of the old gods or to legitamise a claim to land.
Stones circles, put quite simply, are rings of standing stones, although not all of them are cicular, many being eliptical. Many have definite layout plans and often stone circles in one region share a similar style, e.g. Cork features many axial stones circles, where a recumbent stones faces an apparent entrance into the circle (see Drombeg (County Cork)).
They are the most well known of megalithic monuments and the ones most likely to capture anyone's imagination. Many theories exist about the original purpose of these enigmatic structures, the most popular (and at times most controversial) one is that they were built as astronomical observatories, many having apparent solar alignments with the sunrise and sunsets at the solstices and equinoxes. Lunar and star alignments have also ben noted.
No matter what the exact purpose it is certain that they played a significant role in the ritual or religious lives of the builders. One thing that nearly everyone has in common is that they are located in the most dramatic of places, usually offering unrivalled views.
Quite often other monuments, such as alignments, cairns, boulder burials or outliers, are to be found in close proximity to stone circles.