Poincaré disc model
The Poincaré disc model for ${\mathbb{H}}^{2}$ is the disc $$ in which a point is similar^{} to the Euclidean point and a line must be one of the following:

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a diameter^{} (excluding its endpoints^{}) of the unit circle;

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an arc (excluding its endpoints) of a circle such that it intersects the unit circle at two distinct points and the two circles are perpendicular^{} at both intersection points.
The Poincaré disc model has the drawback that lines in the model do not resemble Euclidean lines; however, it has the advantage that it is angle preserving. That is, the Euclidean of an angle within the model is the angle measure in hyperbolic geometry. For this reason, this model is also referred to as the conformal disc model. (See the entry conformal for more details.)
Some points outside of the Poincaré disc model are important for constructions within the model. The following is an example of such:
Let $\mathrm{\ell}$ be a line in the Poincaré disc model that is not a diameter of the circle. The pole of $\mathrm{\ell}$ is the intersection of the Euclidean lines that are tangent (http://planetmath.org/TangentLine) to the circle at the endpoints of $\mathrm{\ell}$.
Note that this matches the definition of pole for the BeltramiKlein model. Also, poles are important for the same reason that they are important in the BeltramiKlein model: Given a line $\mathrm{\ell}$ that is not a diameter of the Poincaré disc model, one constructs a line perpendicular to $\mathrm{\ell}$ by considering Euclidean lines passing through $P(\mathrm{\ell})$. Thus, two disjointly parallel lines $\mathrm{\ell}$ and $m$ that are not diameters of the Poincaré disc model, one constructs their common perpendicular by connecting their poles. It is actually much easier to do this construction by finding the poles of the two lines, finding the common perpendicular with respect to the BeltramiKlein model, then converting the common perpendicular to the Poincaré disc model. See the entry on converting between the BeltramiKlein model and the Poincaré disc model for more details.
In all pictures in this entry from this point on, blue segments are lines in the BeltramiKlein model, and red arcs are lines in the Poincaré disc model.
Below is a picture of two disjointly parallel lines $\mathrm{\ell}$ and $m$ in the Poincaré disc model, neither of which is a diamter of the unit circle:
Their poles can be found:
The common perpendicular with respect to the BeltramiKlein model can be found:
From this, the common perpendicular $n$ with respect to the Poincaré disc model can be found:
Title  Poincaré disc model 

Canonical name  PoincareDiscModel 
Date of creation  20130322 17:06:56 
Last modified on  20130322 17:06:56 
Owner  Wkbj79 (1863) 
Last modified by  Wkbj79 (1863) 
Numerical id  25 
Author  Wkbj79 (1863) 
Entry type  Definition 
Classification  msc 51M10 
Classification  msc 5100 
Synonym  conformal disc model 
Related topic  ConvertingBetweenTheBeltramiKleinModelAndThePoincareDiscModel 
Related topic  ConvertingBetweenThePoincareDiscModelAndTheUpperHalfPlaneModel 