# QuickstartΒΆ

CCL is structured around `Cosmology`

objects which hold the cosmological
parameters and any tabulated data associated with a given cosmology. The
library then provides functions to compute specific quantities of interest.
See the full API documentation through the `pyccl`

module and submodules for more details.

Further, CCL follows the following conventions:

- all units are non-h-inverse (e.g., Mpc as opposed to Mpc/h)
- the scale factor
`a`

is preferred over redshift`z`

as a time coordinate.- the
`Cosmology`

object always comes first in most function calls- argument ordering for power spectra is
`(k, a)`

This example computes the comoving distance and HALOFIT non-linear power spectrum using the BBKS transfer function:

```
>>> import pyccl
>>> cosmo = pyccl.Cosmology(Omega_c=0.25, Omega_b=0.05,
h=0.7, n_s=0.95, sigma8=0.8,
transfer_function='bbks')
>>> pyccl.sigma8(cosmo) # get sigma8
0.7999999999999998
>>> z = 1
>>> pyccl.comoving_radial_distance(cosmo, 1./(1+z)) # comoving distance to z=1 in Mpc
3303.5261651302458
>>> pyccl.nonlin_matter_power(cosmo, k=1, a=0.5) # HALOFIT P(k) at k,z = 1,1
143.6828250598087
```

See Notation, Models and Other Cosmological Conventions for more details on the supported models for various cosmological quantities (e.g., the power spectrum) and the specification of the cosmological parameters.

A comprehensive set of examples showcasing the different types of functionality implemented in CCL can be found here.