I just wanted to clear up titles and forms of address in Downton Abbey for those who may not be up to scratch. I read a post earlier assuming baron and baronet were the same thing, so best to clear it up.
The highest noble title, I am excluding royalty, is Duke. We have met only one Duke in Downton Abbey, the Duke of Crowborough in Season 1. A Duke should always be addressed as Your Grace formally, or 'Duke' socially. For example, You should never address the Duke of Westminster as Lord Westminster.
The second highest title is Marquess. We have met at least one of these in Downton, namely the Marquess of Flintshire, who is styled Lord Flintshire. If you ever meet a Marquess, he should be addressed as Lord 'Title' (I.e. You would address the Marquess of Bath as Lord Bath).
The third highest title is Earl. Our most famous Downton Earl is Robert Crawley, Earl of Grantham. Like Marquesses, always address them as Lord 'Title'. The daughters of Earls are always styled Lady (I.e. Lady Edith Crawley). Unlike Dukes and Marquesses, the sons of Earls are not styled Lords, unless they enjoy a courtesy title.
The next noble rank is Viscount. We all know Anthony Foyle who is Viscount Gillingham. Now I hope you're seeing that we never address a titled peer by their family name, but instead by their title.
For instance you would never address Cora Crawley as Lady Cora or Lady Crawley, but as Lady Grantham, because she is the Countess of Grantham. Nor would you address Lord Gillingham as Lord Anthony or Lord Foyle because he is Lord Gillingham.
The next rank is baron. Hopefully you recall Lord Aysgarth who tried to seduce Mrs Levinson in the 2013 Xmas Special. A baron is always styled Lord.
We then have baronets and knights who never carry the style Lord. Examples are Sir Anthony Strallan and Sir Richard Carlisle who are both baronets/knights. You would never address them as Lord Anthony or Lord Carlisle, always Sir, followed by their Christian name.
I hope this helps clear up and explain the range of titles and forms of address. Any questions, please let me know :)