The Ames guide is a classic comic’s tool. Also used for other forms of calligraphy, from drafting to decorative, it’s a simple variable ruling guide to render lines for lettering.
I’ve made a simple short video to watch here, and you can read the printed instructions most of them come with at the bottom of this page. If you would like to buy one, try your local art supply shop first, if that fails then google!
Dustin Harbin has posted a great photographic tutorial for using it too; The great letterer Todd Klein, who’s graphic I clipped for the thumbnails here – it’s lovely! – has a great instructional posted here; Another in cartoon form here on the Drawing Words & Writing Pictures site, from where I borrowed the hansom drawing of the Ames on the left! The book is the brain child of Jessica Abel & Matt Madden, I don’t know who drew the Ames though, that would one of them or someone on this page. All those sites are fantastic resources self directed learners should investigate!
A separate post about how to letter, and how I do so for my comics is coming.