@17stamps,
Quote:
What do you mean by "solo on piece, postal rates"?
One (solo) postage stamp postally used and still on a shipment (piece), which is franked according to the proper postal rate - collected according to the numbers of the Michel stamp catalogue. Some postage stamps have been used to cover fiscal rates, i.e. taxes and fees not relevant to mail service, which is why I use the terminology "piece".
Most postage stamps are issued to accomodate for actual postage rates, some postage stamps are issued as "supplemental" stamps, they were never intended as solo franking of a shipment - those I collect "on cover", i.e. postally used any witch way.
It is highly important that the postmark is fully readable - the date and place, to know whether the postage stamp was postally used in the correct period.