Ina house where would you find an architrave?

Architrave is commonly used as "skirting board" in houses. It can also be used to frame doorways or windows, or to create picture rails. It is a simple length of material (usually wood or UPVC) that has a taper to one side that runs end to end. More complex types of architrave might appear "coved", or might be formed on more than one side. Look for architrave in builders merchants or D.I.Y shops, or look it up online if you want to know what the different kinds look like.