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Given the number of folders, fixed blades and SAKs I own, the combinations are endless. It all depends on the season, weather, agenda, and mood.
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This^^^
This thread begs the question of the ages - perplexing mankind since flint and obsidian. If I may, I prefer to have three categories rather than three specific knives which, has been said, depends on variables.
FIXED - nothing compares to a fixed blade, be it large or small. Fixed blades are stronger (for given size/weight), are easier to keep clean and do not require the fiddling work of maintained that some folder require. The sheath is key to a fixed blade being EDC-able.
FOLDER (Light) - Be it a classic slip joint or small Spyderco/Ontario RAT 2 slicer, a small lightweight FFG slicer is always useful.
FOLDER (Heavy) - OK, so it may not be necessary if you have a fixed blade but if you've got the britches for it (summer weight tropical wools need not apply) who among us does not enjoy rocking an AD-10 or 4-Max?
SAK - always needed; always welcomed, Sheeple friendly (if you care) but so much so that they should not even count among the three thereby knocking out another choice; IOW, three PLUS a SAK is fine with me ... but if need be, I would certainly want a SAK as one of the knives on my person.
MULTI-TOOL - See my comments on SAKs above. If I'm carrying a multi-tool, I probably don't need much else - but what fun would that be?
LOANER - If you're on BF and reading this thread - you're a knife guy (or gal) and others around you probably know that. That means they will ask to borrow your knife and, if you even get it back at all, will return it abused and used for something that a knife was not meant for. Loaners should not be $5 gas station knives that will hurt someone but my favorites do not get loaned.
I said all of that to say that depending on the planned activities, mode of attire (heavy knives need heavy belts and heavy pants) I do like the following:
AD-10 or Recon-1 or ZT630 or Emerson
de jour
Spyderco Pacific SALT serrated
Small fixed blade (for EDC) certainly go big or go home if hunting, camping or otherwise afield.
Not counted as past of the three are SAKs and MTs ... they just are.