# Conventions¶

## Common variable names¶

Unless otherwise stated (in function or class docstrings), this package tries to adhere to using standard variable names for function and class arguments, and in example code. For shorthand, the variable w is used to represent arrays of phase-space coordinates (e.g., positions _and_ velocities). When representing only positions, the variable q is used. For just velocities or momenta, the variables p or v are used.

## Array shapes¶

The arrays and Quantity objects expected as input and returned as ouput throughout Gala have shapes that follow a particular convention, unless otherwise specified in function or class docstrings.

For arrays containing coordinate or kinematic information, axis=0 is assumed to be the coordinate dimension. For example, for representing 128 Cartesian 3D positions, the object would have shape (3,128).

For collections of orbits, arrays have 3 axes. As above, axis=0 is assumed to be the coordinate dimension, but now axis=1 is interpreted as the time axis, and axis=2 are the individual orbits.

## Energy and momentum¶

The gala documentation and functions often refer to energy and angular momentum and the respective quantities per unit mass interchangibly. Unless otherwise specified, all such quantities – e.g., energy, angular momentum, momenta, conjugate momenta – are in fact used and returned per unit mass.