| Verdana. It was designed for web use and projection happens at the same resolution, so it is generally very readable on screen. I use plain for stuff in the liturgy that the pastor says, and bold for anything the congregation says or sings.
I've found Arial to be too tight for good readability in our sanctuary -- Verdana is a bit wider at a given pointsize. But I'm intrigued by some of the other options people have listed. |