Wales and Scotland will make history by facing each other in an autumn Test in 2018.
The Celtic nations usually only face each other in the annual Six Nations and host autumn Tests against touring teams.
They have played each other only once before outside a tournament - a 2003 World Cup warm-up Wales won 23-9.
The 'Doddie Weir Cup' will be at stake in honour of the former Scotland lock who has Motor Neurone Disease.
Scotland will visit Principality Stadium in Cardiff on Saturday, 3 November.
The two teams' 2018 Six Nations kicks off on Saturday, 3 February when Wales host Scotland at the same venue.
Autumn's game will be played outside World Rugby's Test-match window.
Major southern hemisphere teams such as Australia and South Africa have often fulfilled such fixtures with Wales.
Welsh Rugby Union chief executive Martyn Phillips said: "We need four international matches in our autumn calendar in order to provide vital funds for the future sustainability of the game in Wales."
Scottish Rugby chief executive Mark Dodson said: "The new fixture in November will give Gregor Townsend a chance to put the team through their paces ahead of our 2018 Autumn Test series.
"The introduction of the 'Doddie Weir Cup' reflects the warmth with which he is held across the rugby community."