General understanding of Shields

Hello,
I hope I'm writing this in the correct forum as this is a question about project planning but also about multiplexing ...
Let's see, from what I've seen in the shops, there are a lots of shields as pre-made extensions like SD card readers, LCDs, communications panels etc.
All of them plug into the digital and analog pins and seem extendable, meaning that I can plug the SD card reader into the mainboard and plug the LCD on top.

How can I handle this myself?
I would need a board to control my relays and to connect my external sensors on top of my arduino one and on top of that the SD card reader and on top of that the LCD.

But does that mean that my selfmade shield needs some kind of multiplexer in order to distinguish between digital signals for the relays and signals for the LCD or is it just not possible to use the same pins as the LCD shield on top?

Thanks for your clarifications and if you have a tutorial on design principles for a new shield, I wouldn't look away (I'm somehow missing this information here)

Robert

A tutorial can be found here - http://aaroneiche.com/2010/06/24/a-beginners-guide-to-making-an-arduino-shield-pcb/ -

For shields check - shieldlist.org -, there is information about which pins are used by many shields

or is it just not possible to use the same pins as the LCD shield on top?

There is no magic being performed by shields, it's just a way to mechanically and electrically connect external components to the basic Arduino board. The biggest challenge when trying to plan out what preassembled shields one might use in a project it to determine if there is any conflict in what I/O pins the various shields use, as they can't generally be shared. An exception might be shields that are using I2C communications with the arduino, as I2C data and clock pins are designed to be shared among 'slave' devices. Also when designing a multiple shield project it's important to carefully study the power requirements of all the shields together to determine if the basic arduino board can supply the required current or if a different powering method is going to be needed.

Lefty