def return_names(cols,tickers):
    """This function takes ticker(s) and column(s) and defines the names
    of the combination of both.

    Inputs:
    -------
    cols (list|string): column names (e.g. 'Adj Close', 'Close', 'High').
    tickers (list|string): ticker names.
    
    Outputs:
    --------
    col_names (list): names of the columns for the given cols and tickers
    """
    # Give the correct type:
    if isinstance(cols, str):
        cols = [cols]
    if isinstance(tickers, str):
        tickers = [tickers]

    col_names = [col+'_'+ticker for col in cols for ticker in tickers]

    return col_names