A sketch is not a finished drawing. The purpose of sketching anything is to capture the moment while it's there, or to record a visual idea which might be developed more fully later. The sketching process tends to be rapid, and the speed of drawing brings its own character to the work. Sketches aren't meant to be detailed, accurate studies; they're impressions.

Sketch books come in all shapes and sizes, of course, but I tend to favour the smaller ones as they can be carried in a jacket pocket or shoulder-bag, or they can sit amongst a pile of novels next to a cup of tea where they're within easy reach. All that's required is to find a blank page and something to draw with.

Usually, I use an ink pen. Again, this is partially for convenience. There's no need to sharpen it, unlike with a pencil, and it won't spread sooty fingerprints over the furniture unlike charcoal. Charcoal is a lovely medium to work with in the studio, but in an armchair in front of the TV it can be a menace - and this is where I tend to do most of my sketching these-days.