def specify_grid(grid):
    """Specify the grid to search on (`F` for use of xorca_brokenline).

    Parameters
    ----------
    grid : str
        Name of the grid (T, U, V, F).

    Returns
    -------
    S : str
        Name of the dummy variable living on the specified grid.

    """
    # make sure the letter describing the grid is lowercase
    lgrid = str(grid).lower()

    # define name of dummy variable (e1?)
    if (lgrid == 't') or (lgrid == 'u') \
            or (lgrid == 'v') or (lgrid == 'f'):
        var = 'e1' + lgrid
    else:
        raise ValueError(str(grid) + ' is not a valid grid-name')

    return var