More accurately, a "penknife" (which are always legal because they are exempt from the weapon law) would be any hinged knife that the blade folds into it's handle, that is not a true switchblade. A "switchblade" as defined under state law (CR 4-105) has some kind of a switch or activator device that is incorporated into the knife's handle. So called "Assisted opening" knives have springs, but no switch. You still have to touch the blade part to open it. That's why they are legal in most states including Maryland. Strangely enough, butterfly knives are also legal in Maryland because they too count as penknives.
One of the big myths in Maryland is that there is a limit to how long the blade can be. Not true, as stated in Mackall v State above. It can be a Spanish navaja with a 10 inch blade and it still counts as a penknife. When you get down to it, most folders are legal in Maryland. The law is actually quite relaxed; courts have even taken cops badges away for arresting people with a penknife. It's just when you get into concealed fixed blades (open carry of anything is legal) that it get's dodgy.