In a role playing game called Wizards of the Coast they describe Ley Lines on their website to members...
Regent mages who control sources of mebhaighl can command arcane realm magic, the most powerful arcane magic available to human- and elven-kind. In order to cast realm spells, however, a regent mage must draw on the power of her source holdings. Arcane realm spells require that the caster have a source holding in the province in which the spell is cast. The level of the source holding may limit the realm spells that can be cast by the regent in the province. Ley lines provide a means of accessing a regent's source holdings in other provinces to allow the regent more flexibility in the casting of realm spells.
Ley lines are mystic conduits that allow a true mage to tap the mebhaighl from the source where it collects and transport it to a province in which he wishes to cast a realm spell. A ley line creates a magical link between two provinces. For the purpose of casting realm spells, the caster may use the highest level source that he claims from either of the two connected provinces. If either of the two connected provinces is connected to a ley line, then the caster may use the highest level connected source anywhere in the ley network.
A mage can only forge ley lines from a province in which he holds a source (the initial province), but the mage does not have to have a source in the terminal province to which the ley line connects. Ley lines can be forged over any overland distance. A ley line cannot extend over more than 150 miles of water. A ley line only connects two provinces (the initial and terminal provinces). A ley line that simply passes through a province cannot be used to cast realm spells.
Ley lines, like rivers, curve and meander to accommodate terrain, but for game purposes are assumed to define a straight line between the center of the initial and terminal provinces. The length of a ley line is the number of provinces it touches, not counting the initial province. For example, a ley line between two adjacent provinces has a length of 1.
To construct a ley line, the caster must spend a significant portion of a month in the initial province, and a brief time (one day) in the terminal province. During this day, the mages goes into a trancelike state during which she loses track of her surroundings and is largely defenseless. If disturbed during this period, the mage's forging attempt automatically fails. Powerful regent mages generally travel with quiet guards or trusted friends pledged to protect them during this period of concentrated effort. When a ley line is forged, all true mage source regents in the provinces though which it passes are aware of the change in the flow of mebhaighl through their realms. These regents are aware of the direction of the mebhaighl flow and are aware of whether the line begins, ends, or simply passes through their province. Source regents in any province through which a ley line travels may use their source levels (and RP) to aid or hinder the forging of the ley line. Once created, however, a ley line cannot be detected or destroyed except through the use of realm spells cast for that purpose.
Once created, a ley line costs nothing to maintain. However, the use of a ley line to perform any realm spell increases the regency point cost of that spell by the number of provinces crossed by the ley network that connects the province to the necessary source. Ley lines are a non-transferable domain asset. When a regent dies, her ley network is destroyed. Ley lines cannot be used or invested to another caster. The only exception to this rule is the use of the ley lines by the regent's lieutenant as part of a lieutenant domain action.