Bells and whistles
What's the meaning of the phrase 'Bells and whistles'?
Attractive additional features or fittings
What's the origin of the phrase 'Bells and whistles'?
'Bells and whistles' appeared many times in 18th and 19th century texts in literal references to warnings or promotional events. These contexts included citations about fire engines, the Salvation Army, circuses; anyone in fact that was trying to draw attention to themselves might do so using a bell or a whistle.

We are writing in COBOL under DOS and [our system] will have all the bells and whistles.
It wasn't long before computer salesmen got in on the act and started to offer computers with 'all the bells and whistles'. An example from that context comes from the US computer magazine Computerworld, August 1976:
Before even considering bells and whistles, a user should look at the plain vanilla system and see just how operationally sound it is.
So why was 'bells and whistles' chosen as synonymous with 'the full complement of accessories'? After all, the expression bears little relation to the earlier 'making a lot of noise' meaning. It is possible that, by naming two items with a similar function, the expression just suggested excess, something akin to 'belt and braces'.
The derivation of 'bells and whistles' isn't definitively known so, while it isn't my preferred method to speculate, there isn't much option in this case but to offer plausible derivations and continue to look for better evidence.
The coiners of the present meaning of the term probably weren't thinking about fire engines etc. when they adopted the new meaning. One possible source for the phrase is fairground organs. These were somewhat like what a one-man band would resemble if he formed an army. It hardly does them justice to call them musical instruments; they were orchestral automatons on an industrial scale and bristled with every form of instrument that could be banged, shaken or blown. The excess implied in 'bells and whistles' fits well with the image of fairground organs.

