and yet more explanation
If you have a crucifix at home and out of personal devotion you kiss it, this is also properly called a veneration of the cross. The customary place to venerate the cross is the feet of the corpus. During a Good Friday service, if you have an open wound or sickness that can be communicated by the mouth, you may venerate the cross by placing your right hand upon the feet of the corpus for a moment. I do this just the hand
There is one exception to rule above when a cross can properly be used in place of a crucifix: any cross that contains a relic of the true cross - that is an actual piece of the cross Jesus Christ died upon at Calvary - does not have to have a corpus upon it. The correct way to venerate this type of rare cross is to kiss the glass reliquary behind which the fragment is visible. If no reliquary is visible, the exact spot for veneration is not important, though kissing the foot of the cross is customary.