In this modern day and age of guitar technology - there are many innovative players and genres that are constantly pushing the boundaries of guitar performance and technique - as such, guitar manufacturers are in turn producing new and exciting instruments to accompany them!
The purpose of adding an extra string/s to your instrument is not to confuse and baffle (although that may well be the case at first) - but to provide an expanded sonic range that your guitar is capable of producing and that you, the player, is capable of exploring.
Most Electric Guitars come built with a 24.75" (Les Pauls, SGs) or 25.5" (Stratocasters) Scale Length which is the distance between the Nut and the Bridge of the guitar. A 7 or 8 string guitar will typically be engineered to have an extended scale length ranging from anywhere between 25.5" to 30" depending on manufacturer and build. As stated - many 6 string guitars will have a 25.5" scale, however tuning down on that will in turn sacrifice the range for your lead work on the 1st string.
The two main aspects that differing scale lengths influence are overall feel of the strings and intonation. A longer scale length will generally provide more tension and a 'punchy' feel, whilst a shorter scale length will give you less tension and a looser feel. Similar to how you would normally associate a Les Paul with more of a 'beefy' sound whilst a Strat would provide a bit more 'twang'. In regards to intonation, which is how in-tune your string stays the further you go up the neck, typically the longer your scale length the better your string will stay in tune (given that it is setup correctly). Of course there are many players that swear by downtuning a 24.75" guitar, however a longer scale length can help!
Another unfortunate pitfall of tuning a standard 6 string down is that the nut (what the strings sit in at the join between the neck headstock) slots may not be built to handle the added thickness and tension that thicker strings will inevitably add. The last thing you want is a broken nut in the studio or even worse, on stage! A 7 string nut will have a wider string slot and likely a wider tuning post hole to accomodate a thicker string which will eliminate needing to irreverisbly modify your beloved 6 string.
One other aspect to consider is that 7 and 8 string guitars will often have Pickups specifically voiced for lower tunings. A 6 string downtuned may often sound muddy and get lost in the mix, whilst the pickups designed for the job will ensure that your carefully crafted tone is not lost and that it maintains clarity.
Like any guitar, there is no wrong or right way to go about it, as at the end of the day your fingers and ears like what they feel and hear. But getting the right gear for the right job can always help!
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