There are some fashion items that are so rudimentary that I can never remember to buy them and when I really need them in the moment and tell myself to order them,
I forget as soon as I see a bright shiny object to order instead. Know what I mean? In this case, the thing I always forget to buy but always need (this time of year especially) is long-sleeve T-shirts—both solid and striped. The ones I wear the most usually have a crew neck, and I find that owning a combination of fitted and oversize ones will do you the most service.
My observation is that long-sleeve T-shirts often get overshadowed by short-sleeve tees, sweatshirts, and sweaters. But when you need to layer, or when you need something that's not too warm but not too light, your best bet is a long-sleeve tee. In case you need reminding of just how essential they are to an outfit, I found a whole slew of examples for you. And in case you need to stock up on them yourself, I'm sharing 16 of my current favorites, most of which are under-$100. Scroll to be convinced.