"half n half"
Heavy cream (such as whipping cream) is about 36% fat. Half-n-half is half that, about 18%. Whole Milk is about 4%. Sweet Butter (unsalted) is about 80% fat. Margarine, also, is about 80% fat and 20% water.
SUBSTITUTIONS: Sometimes you can, sometimes you can't. For example, butter and margarine have the same fat and water content, but you can't make a beurre blanc with margarine because it lacks the proteins in butter (which act as emulsifiers). Also, if you want to make a "pan sauce" from the deglazed fond in the pan with some "liquid" like water or wine and some butter - it has to be butter to get the "creamy" texture .. margarine just gives you an "oil slick" - unless you want to throw in an egg yolk to act as the emulsifier. Although, you could use 1/4 - 1/2 the "liquid" and use heavy cream and wind up with something similar as far as texture and "mouth feel".
Sometimes - you can substitute. I've got a recipe from my Grandmother for a chocolate sheet cake ... and replaced her "oleo margarine" with butter flavored Crisco and it's moister, doesn't dry out as quickly, and has a better butter "flavor". I normally don't mess with Grandma's recipes .. but I spent a good month researching before I monkeyed with this one.
