What are the base and quintessential cards for?

Sorry if this sounds like a VERY stupid question but lately some members have read using these cards for complementation and I have no idea how to "get" them (is there any calculation involved) and what their meanings stand for. Could anyone more experienced tell me, please? Thanks!

Ana

Tirage en Croix

• 1. the querent
  2. something/one that opposes the querent
  3. something that helps - will help - the querent or gives assistance to what opposes the querent
  4. the direction in which the querent is going/the answer to the question
  5. quintessence: you choose it by adding up the numbers of the four other cards and reducing it (by theosophic reduction) to a number between 1 and 22 - the corresponding card to that number is the quintessence card, that synthesises your whole reading.

Base card, Shadow card, Da'at card is also called the bottom card. the card on the bottome of the deck.

Umbrae has linked you to the Tirage en Croix, which is where the idea of quintessence in a tarot reading comes from.

The word "quintessence", and its concept in tarot, is derived from alchemy. In Western alchemy, there are 5 elements: Air, Fire, Water and Earth, and a fifth - Quintessence (meaning "fifth essence") - which is a mysterious element that contains the essence of all the others but also contains the Spirit. And so in a reading, the quintessence card - derived numerically from the other cards - is the sum of the essence of the other cards, which also brings an overarching spiritual direction to the reading. Given its nature, it can't be read in isolation from the other cards, but neither is it a card like any other. In practice I often find that it is the same card (always a Major of course) as one of the other cards, so that you can easily see where the throw is weighted.

I have to add that in the Tirage en Croix, at least, the quintessence card is not simply a complement, but is a vital part of the spread. I use the quintessence card with other spreads, but only where it makes sense to make it as an important part of the reading, otherwise not.

The Base or Shadow card is the card at the bottom of the deck, which can give you some information about things that are withheld, or shadowed, in the reading.