Users frequently need to enter commands while typing (e.g., for formatting or UI control). These commands usually involve either pointing to GUI controls, which is slow because it requires moving the hand to the mouse, or pressing modifier-based hotkeys, which can be awkward because it requires reaching for the modifier key. Previous research has proposed chording as an alternate mechanism, in which commands are expressed by chording letter keys. Letter chords can be reliably recognized and learned, but previous work does not identify which types of chords are best, and does not assess the performance of letter chords in realistic tasks. We carried out two studies to provide this information: the first measured speed, accuracy, and preference for different types of letter chords; the second evaluates letter-chord performance compared to hotkeys. Our studies identify several features that are important for usable letter chords, and show that in two different tasks, letter chords significantly outperform modifier-based hotkeys by 8-15%.