The Euclidean plane is the manifold $M=\mathbb{R}^2$, with coordinates $(x_1,x_2)$, together with the natural Riemannian metric
$$ g=dx_1\otimes dx_1+dx_2\otimes dx_2. $$It is, therefore, a Riemannian manifold.
Seen as a plain manifold we can consider that $M$ is endowed with a natural linear connection $\nabla$, such that $\nabla_{\partial_{x_i}}\partial_{x_j}=0\partial_{x_1}+0\partial_{x_2}$. The induced principal connection on the frame bundle is given by the 1-form
$$ \omega= \begin{pmatrix} c_{11}&c_{12}\\ c_{21}&c_{22}\\ \end{pmatrix}^{-1}\cdot\begin{pmatrix} dc_{11}&dc_{12}\\ dc_{21}&dc_{22}\\ \end{pmatrix}\in \Omega^1(\mathbb R^2,\mathfrak{gl}(2)) $$at
$$ f= \begin{pmatrix} c_{11} & c_{12}& x_1\\ c_{21} & c_{22}& x_2\\ 0 & 0& 1\\ \end{pmatrix}\in FM $$Here it is shown how to construct $\omega$ is constructed from an arbitrary $\nabla$.
On the other hand, if we think of $M$ as a Riemannian manifold we can consider the Levi-Civita connection $\nabla_{LC}$. Since the metric is constant, the covariant derivative $\nabla_{LC}$ coincides with the natural covariant derivative $\nabla$, so it induces the same connection $\omega$ on $FM$.
But the metric $g$ also specifies a orthonormal frame bundle $OM$ (see here why). The elements of this principal bundle are
$$ f= \begin{pmatrix} c & -\sqrt{1-c^2}& x_1\\ \sqrt{1-c^2} & c& x_2\\ 0 & 0& 1\\ \end{pmatrix}\in OM $$with $c\in [-1,1]$.
Since
$$ \omega|_{OM}=\begin{pmatrix} c&\sqrt{1-c^2}\\ -\sqrt{1-c^2}&c\\ \end{pmatrix}\cdot\begin{pmatrix} dc&\frac{c}{\sqrt{1-c^2}}dc\\ \frac{-c}{\sqrt{1-c^2}}dc&dc\\ \end{pmatrix}= $$ $$ =\begin{pmatrix} 0&\frac{dc}{\sqrt{1-c^2}}\\ -\frac{dc}{\sqrt{1-c^2}}&0\\ \end{pmatrix} \in \Omega(\mathbb R^2,\mathfrak{o}(2)), $$according to Proposition 4.7. in [vicenteBundles](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwj-w6a2xYj7AhVJ3hoKHVDpAMQQFnoECAsQAQ&url=http%3A%2F%2Fwww.mat.ucm.es%2F~vmunozve%2FNotas-fibrados.pdf&usg=AOvVaw2EmcybsLZEb16epRLqS8z8), this connection can be reduced to a connection on the orthonormal frame bundle determined by the metric $g$.
If we parameterize this principal bundle $OM$ with
$$ (x_1,x_2,\theta)\mapsto \begin{pmatrix} \cos(\theta) & -\sin(\theta)& x_1\\ \sin(\theta) & \cos(\theta)& x_2\\ 0 & 0& 1\\ \end{pmatrix} $$we obtain the more famous expression for $\omega$:
$$ \begin{pmatrix} 0&d\theta\\ -d\theta&0\\ \end{pmatrix} $$Remember: this 1-form tells us how much the bases at $f$ and $f'$ "fail to be constant" when we pass from the frame $f$ to a nearby frame $f'$, but expressing this mistake with respect to the frame $f$ itself.
The Euclidean plane is a Cartan geometry modeled over $(E(2),O(2))$ , indeed is the Klein geometry $(E(2),O(2))$. Moreover, it is a reductive Klein geometry since
$$ \mathfrak e(2)=\left\{\begin{pmatrix} C & v\\ 0 & 0\\ \end{pmatrix} :C\in \mathfrak{o}(2), v\in \mathbb R^2\right\}=\mathfrak o(2)\oplus \mathfrak p $$We have a natural choice for $\mathfrak p$
$$ \mathfrak{p}=\left\{\begin{pmatrix} 0 & p\\ 0 & 0\\ \end{pmatrix} :p\in \mathbb R^2\right\}. $$With this in mind, remember that the Maurer-Cartan form describes all possible "infinitesimal displacements" of the frame $f$, but from the point of view of the frame $f$ itself (see this). That is, if we pass from the frame $f$ to another frame $f'$, the Maurer-Cartan form at $f$ applied to the "vector" $\vec{ff'}=(dx_1,dx_2,d\theta)$ is a packet of information
$$ A=\begin{pmatrix} 0&-d\theta&cos(\theta)dx_1+sin(\theta)dx_2\\ d\theta&0&-sin(\theta)dx_1+cos(\theta)dx_2\\ 0&0&0&\\ \end{pmatrix}\in \mathfrak e(2) $$Here is encoded, on the one hand, how much have we moved the base point of $f$ to the base point of $f'$ and, on the other, how much have we changed the basis itself. The natural choice of $\mathfrak p$ let us think that the information about the change of base point is in the $v$ part (the projection of the Maurer-Cartan form on $\mathfrak p$), and so the projection of the Maurer-Cartan form on $\mathfrak o(2)$, $\begin{pmatrix}0&d\theta\\-d\theta&0\\\end{pmatrix}$, tell us how much has the basis changed. That is, the same as the connection 1-form of the connection on the orthonormal bundle induced by the metric $g$ (which is the Levi-Civita connection).
To summarize:
In other words, in the orthonormal frame bundle induced by the metric $g$ we consider a displacement from a frame $f=\begin{pmatrix}C & p\\0 & 1\\\end{pmatrix}$ to a frame $f'=\begin{pmatrix}C' & p'\\0 & 1\\\end{pmatrix}$ .
The principal connection $\omega$ induced by the Levi-Civita connection measures the change from $C$ to $C'$ as an element of $\mathfrak o(2)$.
The Cartan connection (Maurer-Cartan form) measures the change from $f$ to $f'$ as an element of $\mathfrak e(2)$. This change can be decomposed like the union of a change from $C$ to $C'$ and a change from $p$ to $p'$. This is reflected in the fact that $\mathfrak e(2)=\mathfrak o(2)\oplus \mathfrak p$. If we focus on the change from $C$ to $C'$ we have the principal connection $\omega$.
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Author of the notes: Antonio J. Pan-Collantes
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