The Black and Tan is commonly served in pubs throughout Ireland, England, and the United States. It is a drink that is equal parts stout and pale ale. The drink gets its name from the colors of the uniforms worn by the British police who fought the Irish Republican Army in the early 1920s. Not surprisingly, that time and those colors do not elicit fond memories for the Irish, so the drink is often called a Half-and-Half in Irish pubs. I happen to like to say Black and Tan myself. :)
Regardless of its title, the drink is made by filling a glass halfway with pale ale (usually Bass Ale, or an Irish favorite, Harp lager gives a lighter touch). The ale poured into the glass slowly and against the side, to prevent a big head from forming. Once the beer has settled, the stout (usually Guinness) is slowly poured over the back of a spoon, which will help keep the darker beer from forming a head, and will also help to keep the beers in two distinct layers.
I had this amber ale on hand which works really great. Guinness is what I always use for black.
Grab a glass and fill it 1/2 to 2/3 full of the ale tilting the cup and pouring the ale down the side of your glass to cut down on foam. Let any foam go down before pouring your stout.
Take the back side of a large soup spoon and carefully and slowly pour the Guinness over. I couldn't shoot the photo as I poured. See the distinct layers? You can also use Blue Moon with Guinness for a Black and Blue :)
Happy St.Patrick's Day!