876,170. Indicating availability of seats. ELECTRIC & MUSICAL INDUSTRIES Ltd. Nov. 29, 1957 [Nov. 29, 1956], No. 36484/56. Class 106 (3). An automatic inventory system comprises a plurality of subsidiary stations each including an availability record and means for testing availability by reference to said record in response to a reservation enquiry signal, and a main station comprising means common to the subsidiary stations for deriving availability record-correcting signals in response to reservation signals from the subsidiary stations, means for transmitting said correcting signals to each subsidiary station at intervals and means at each subsidiary station for updating the respective availability record in response to the correcting signal. The system may employ a teletype channel and may include a plurality of hand sets, connected to the subsidiary stations by the teletype channel. Fig. 1 shows diagrammatically the general layout of an inventory system used by an airline. An enquiry about availability of space made at a subsidiary station or at a sales office connected to the subsidiary station, starts a cycle of operations as follows: by operating the hand set, the agent, connected to 1, obtains a availability reference from 2. If favourable, the reservation is recorded at 2 and re-checked by 4 according to information received from other substations. Cancellation signals are recorded in the same way. At predetermined intervals aggregated reservations and cancellations recorded at subsidiary stations are sent to the main station and recorded by a computer which comprises a space-control inventory 6. When the space control inventoty has manipulated incoming signals, amendment information is compiled and transmitted by means 7 to the subsidiary stations. If, in response to an enquiry the availability testing means 2 indicates that no seats are available on a particular flight, the inventory system has facilities for transferring particulars to a wait-list control apparatus 8 located at the main station. This wait-list control apparatus is arranged for the transmission of signals via 9 to the appropriate reservation offices for contacting passengers if seats become available due to cancellations or other reasons. The apparatus at the subsidiary station or sales office connected with it, may include means 10 for storing passenger records for ultimate transmission to the departure points of the respective flights, so that passengers can be checked into aircraft without delay or can be informed if a schedule change 11, is necessary. A representative arrangement of the communication network of a sub-station and a reservations office is illustrated in Fig. 2. Single circuit teletype channels are shown by dotted lines and multiple circuit channels by full lines. When an enquiry or reservation signal is initiated by the hand-set of a reservation officer, the selector unit S automatically connects the hand-set to the outgoing teletype circuit C1. The signal is set up in the buffer store RL and is read at 50 digits per second on receipt of a signal from the sub-station that one of the input buffers R 1 , R 2 , R 3 . .. has been connected by the selector S 2 in the circuit C 2 leading from RL. The information handling apparatus D of the sub-station, indicated at 2 and 4, Fig. 1, selects signals from the buffer stores R 1 , R 2 , R 3 ... in rotation and after processing, feeds the reply message (if any) to store R 0 . The connection from the selector S 1 to the hand set is retained during the process so that no reply address is needed, the reply message being set up in the register RA, after which the selector S 1 is released. The general layout of the computer is shown in Fig. 4. The space control is a magnetic record 40 in which is recorded space available in all flights within a predetermined time, say 30 or 60 days. The entries on the spacecontrol inventory are on the flight-sector class basis and may have a capacity of 60,000, represented by 600,000 characters. For example, each item may comprise date of flight, flight number, block number, sector code, class code, block position number, total number of unreserved seats, number of provisional reservations, number on wait - list, " on cushion " signal, number of " cushion " seats, whether selection applies, bookings at departure office, bookings at return journey office and off-line bookings. Messages arrive in random order from sub-stations 41 and are sorted out into flight and date order by means 42. The sorted information is transferred into two magnetic tape buffer records A and B, one of which receives only a sorted list of date and flight numbers and the other of which receives all the incoming information sorted and condensed as required. The first tape A is used to operate a selector 43 to cause it to select the appropriate items of the space control inventory record 40 by the mechanism described in Specification 866,048. Items thus selected are then combined with the information recorded on tape B by merging means 44, on a transfer tape which feeds the merged information item by item into the input store 45 of the computer proper. The computer service takes about 30 seconds. The aggregation interval for updating the availability record at each sub-station may be 10 to 15 minutes, thus practically obviating any risk of overbooking.