Eigen defines several typedef shortcuts for most common matrix and vector types.
The general patterns are the following:
Size can be
4 for fixed size square matrices or
X for dynamic size, and where
Type can be
i for integer,
f for float,
d for double,
cf for complex float,
cd for complex double.
Matrix3d is a fixed-size 3x3 matrix type of doubles, and
MatrixXf is a dynamic-size matrix of floats.
There are also
RowVectorSizeType which are self-explanatory. For example,
Vector4cf is a fixed-size vector of 4 complex floats.