raising or lowering it, then striking the element with a padded hammer, which would impact the element against the inked ribbon and paper. The Model 33 printed on wide paper, supplied on continuous diameter rolls, and fed via friction (instead of, for example, tractor feed). It printed at a fixed 10 characters per inch, and supported 74-character lines, although 72 characters is often commonly stated. The Model 33 could operate either in half-duplex mode, in which signals from the keyboard were sent to the print mechanism, so that characters were printed as they were typed (local echo), or in full-duplex mode, in which keyboard signals are sent only to the transmission line, and the receiver would have to transmit the character back to the Model 33 in order for it to be printed (remote echo). The factory setting was half-duplex, but it could be changed to full-duplex by the user. The Teletype Model 33 contained an answer-back mechanism that was generally used in dial-up networks such as the TWX network. At the beginning of the message, the sending machine could transmit an enquiry character or WRU ("Who aRe yoU") code, and the recipient machine would automatically initiate a response, which was encoded in a rotating drum that could be preprogrammed by breaking off tabs. The answer-back drum in the recipient machine would rotate and send a unique identifying code to the sender, so that the sender could verify connection to the correct recipient. The WRU code could also be sent at the end of the message. A correct response would confirm that the connection had remained unbroken during the message transmission. To conclude the transmission, the sending machine operator would press the disconnect button. The receiving machine could also be set up to not require operator intervention. Since messages were often sent across multiple time zones to their destination, it was common to send a message to a location where the receiving machine was operating in an office that was closed and unstaffed overnight. This also took advantage of lower telecommunication charges for non-urgent messages which were sent at off-peak times. The Teletype Model 33, including the stand, stood high, wide and deep, not including the paper holder. The machine weighed on the stand, including paper. It required less than 4 amperes at 115 VAC 60 Hz. The recommended operating environment was a temperature of , a relative humidity of