The reason for this is that the earth has a tilt (I'm sure you remember this from science classes) and the sun passes slighty to the south in the sky. so if your trellis is at the north EDGE of the bed, it will shade nothing (provided you're in the northern hemisphere, of course).

Now you'll realize this only works if you're a flat, wide area with nothing else to overshade your garden, If you don't, try to guess where it'll cause the least shade to other plants, while still getting enough sun for the crops growing on it.

I've often put mine in the center of the bed, running north and south. That way half the garden gets the morning sun and the other half the afternoon sun. If I plant this way, I put root crops, lettuce and all the cooler weather crops on the east side, the heat loving crops on the west. The cool crops enjoy a bit of shade in the afternoon.

Granny

Here is a link that might be useful: Annie's Kitchen Garden

Twenty feet of trellis.....cool! You can do it many ways. It's all according to how much sun exposure the actual spot gets. If it gets maybe 10 hours of exposure, I would do what Granny suggests. That way, you would get the most out of your trellis (both sides) And....if it gets less than that, just don't plant it so congested, and the sunlight can shine thru the trellis material from both directions. It's all good.

EG