APERTURE controls how close the crosshairs must get to an object before an object snap marker displays. You can set APERTURE at the command line anywhere from 0 to 50 pixels, or you can set it in the Options dialog box by adjusting a slider to a maximum of 20 pixels. The initial value is 10, but I find 20 pixels is just about right for me. APERTURE is stored in the Windows registry.

If you're using blocks with attributes, you'll want to know about this one. When ATTDIA is set to 0 (its initial value) and you insert an attributed block, you're prompted to enter attribute values in the command window. When ATTDIA is set to 1, you get a nice, shiny dialog box. Shininess apart, dialog boxes are a better bet because you can fill in the blanks in any order you want; if you do it at the command line and miss a prompt, you have to press Enter over and over until you get back to the one you missed. ATTDIA is stored in the Windows registry, so it's another variable you can set and forget.

DIMASSOC controls how AutoCAD's dimension objects are created. When DIMASSOC is 0, AutoCAD creates exploded dimensions of separate lines, 2D solids, and text. You really don't want to work on DIMASSOC=0 drawings. When DIMASSOC=1, dimensions are referred to as "non-associative" — that means that they are single objects, but they usually don't update if you change an object's size. When DIMASSOC=2 (the default), AutoCAD creates fully associative dimensions. You can set this value at the command line or select the Make New Dimensions Associative check box in Options

You've got to love it when a system variable name gives a clear indication of what it does (ROLLOVERTIPS, anyone?). You'll frequently want to build up a drawing by mirroring already-drawn components of it. That's what the MIRROR command does, but unfortunately, if text or dimensions are included in what you're mirroring, those end up being reversed as well. Having to use a mirror to read the text on your drawing is counterproductive, so you can invoke MIRRTEXT to fix things. When MIRRTEXT is 0 (the default), text is mirrored along with everything else; setting it to 1 means everything gets mirrored but the text still reads the right way around. MIRRTEXT is stored in the drawing.

The pickbox is the little square box that the crosshairs turn into when AutoCAD is prompting you to select something. You control pickbox size by setting a value in pixels for the PICKBOX variable. The initial setting of PICKBOX is 3, which is fine for a lower screen resolution like 1024 × 768. But when you start running at higher resolutions — or when your hair is grayer, your face more wrinkled, and your vision not quite as acute as it once was, a higher setting might be more useful. My old laptop computer ran at 1600 × 1200 resolution, and I had to set PICKBOX to 5 to be able to use it effectively. My slightly newer desktop runs at 1280 × 1024, and PICKBOX=4 is fine for that. But I've just acquired a new tablet PC that runs at 1400 × 1050, and I'm back to PICKBOX=5 for this one. As they say, your mileage may vary.

Sometimes you don't want AutoCAD to start in the same Documents folder where every other program stores its files. Maybe you work on a number of projects and would like a separate desktop icon for each to start in a particular folder. REMEMBERFOLDERS is your ticket to nirvana. By default, it's set to 1, which means it defaults to storing files in My Documents (just plain Documents in Windows Vista). Set it to 0, and it will open from and save to the folder you specify in your desktop icon when you start, and the last folder you visit as you work. REMEMBERFOLDERS is a permanent setting that's stored in the Windows registry (permanent, that is, until you change it to something else).

You can get double-duty from this command. Not only can you key it into AutoCAD, but if you have a dog named Tips, you can use it to teach him a neat pet trick! ROLLOVERTIPS is stored in the Windows registry. (And believe it or not — and I'll bet you won't — there's another new system variable called NOMUTT! Could that be AutoCAD-ese for BAD DOG TIPS!?)

Tooltips themselves are the little text boxes that appear (by default, anyway) when you hover your mouse pointer over a toolbar or Ribbon button, or just about anywhere in a dialog box. Tooltips are really useful when you're getting to know your way around a program, but when you have a sense of where things are, they can get in the way. By default, TOOLTIPS is set to 1, which means they display when you hover the pointer over something. Set TOOLTIPS to 0 if you don't want to see them again. The TOOLTIPS setting is stored in the system registry.

If you work with xrefs, you know the potential for competing layer properties. Sometimes you want your xref to look different from your current drawing so you can tell which is which. You can change layer colors of xrefs in the Layer Property Manager, but whether or not you have to do that every time you open a drawing with an attached xref depends on the setting of the VISRETAIN system variable. When VISRETAIN is set to 0, the layer properties in the xref take precedence, which means you would have to make those changes every time you opened the host drawing. When VISRETAIN is set to 1 (the default value), changes to layer properties you make in the host drawing remain intact every time you open that host drawing; however, those changes have no effect on the external file itself. The VISRETAIN value is stored in the current drawing.