Tavis Allison and Cyclopeatron are musing about whether or not to reveal the target numbers for successful melee attacks to their players.
Dave Arneson has an oft-quoted saying: "Don’t ask me what you need to hit. Just roll the die and I will let you know." Essentially, Dave was advocating for keeping as much of the game mechanics off-stage as possible, thus allowing the players to immerse themselves in their characters and the world, rather than meta-gaming, or tailoring their play to the game mechanics.
While I concur with Dave's approach, and have been a vocal proponent of keeping much of the in-game mechanics behind the DM screen, both Tavis and Cyclopeatron make compelling arguments for (sometimes) revealing the to-hit numbers to the players.
The attached 'Percentage Index' table has been sitting on my computer desktop for several months. I think I came across it in one of my visits to the Original Dungeons and Dragons discussion boards. It seems rather Arnesian: rumor has it that, before the d10 and d20 became widely available, probability tables were constructed using the available dice-of-the-day, d6's. The above table allows you to calculate a d20/percentage probability from a throw of 2d6.
Dave Arneson was reportedly a fan of percentage systems. In my mind, I imagine this probability table hanging from Dave Arneson's DM screen, via a paper-clip, and, after calculating the percentage chance to hit, Dave peers over his screen and is saying, "Don’t ask me what you need to hit. Just roll the die and I will let you know."