"Layoff" is commonly believed to be a bookmaker's sole method of wagering. It is any bet made by one bookmaker with another in an effort to achieve what the bookmaker feels is a desirable balance or ratio of wagering necessary to ensure a profit. A layoff is not used just to achieve an even ratio of wagering, but rather to produce a wagering ratio that the particular bookie believes will be most advantageous for profit in a given sports event. One version of a layoff is called "middling," whereby a bookie will take advantage of the fact that two other bookies may have substantially different lines on a game, creating the possibility of winning both bets. Bookmaking personnel include office workers, who take bets by telephone; street agents, who take bets at locations other than the bookie's office; and "beards," who make contacts with other bookies for layoffs. Numerous types of equipment may be used by bookmakers to avoid detection: a "blue box" that enables the user to simulate a touch-tone signal and use tool-free 800 WATS telephone numbers without charge or a toll record, devices similar to call-forwarding equipment or off-premises telephones, and devices that warn bookies when someone is approaching or entering the premises.