Was Launched by the GPO in 1979, in development it had been code named CDSS1.

The first incarnation was a tall equipment cabinet with 7 shelves.

The top or control shelf (A) needed 5 Cards for the computer 4 cards for handling the TDM buses and another 3 cards for tone generation, conferencing and clock generation.

The CPU cards where on the left side of the shelf

Each of the 5 Line shelfs had 6 card slots for 4 port extension cards and 4 Slots for 2 port exchange line cards. There was also a "Shelf Mux" card and the option of a fall back relay card.

The bottom Shelf Held the power supplies and ringer

By 1982 the Monarch 120B compact was available in a desk top configuration, and the tall cabinet Monarch 120 was now called the Monarch 120A. The Monarch 120A could be upgraded to a 120B for £650 see Monarch Application note 10

The processor cards were on the right of the shelf

By 1992 the CD range was available, and is described in Monarch CD Range System Installation commissioning and maintenance Item code 375897

The Monarch system exists in a number of forms. Whilst many items are common to more than one variant, some parts will only fit or be associated with specific-generics.

Originally Monarch was launched in an upright, floor standing, stone coloured cabinet, designated Monarch 120. It is no longer available in this form although existing systems may be upgraded to provide increased capacity or enhanced facilities. The enhanced system was redesignated Monarch SE220 but currently it is designated CD220.

Monarch 120 Compact was introduced as a table top version of the original Monarch. In its largest size it comprised two cabinet modules but could be installed as a single cabinet, the additional cabinet being added at a later date to cater for growth. It is no longer available in the original stone colour, except for growth, but the system was relaunched in the new grey livery and with enhanced features. In this form it was designated Monarch SE 120 but it is now designated Monarch CD 120. The previous models can be upgraded to provide the same features as Monarch CD 120.

The Monarch 250C was introduced to provide greater capacity and gives advances in both hardware and software. It was housed in a floor standing, upright cabinet, similar to that used on the original Monarch. It is ready identifiable by its additional top grill and was available in the stone livery. The system was subsequently enhanced and relaunched as

the Monarch SE250 and is available in grey. This version is now redesignated the Monarch CD250.

A larger version of the Monarch SE250 was introduced. This is an upright two cabinet system available in the grey livery. It was originally designated the SE440 but is now called the Monarch CD440.

A dual processor version of the Monarch, called IT440, was also introduced. The availability of the system was enhanced by the provision of dual processors and zoned power supplies.