Haddington was a royal burgh that returned one commissioner to the Parliament of Scotland and to the Convention of Estates.
After the Acts of Union 1707, Haddington, North Berwick, Dunbar, Jedburgh and Lauder formed the Haddington district of burghs, returning one member between them to the House of Commons of Great Britain.
References
Haddington (Parliament of Scotland constituency) Wikipedia(Text) CC BY-SA