Easily-confused!
If your specimen is a:and has stem/​leaf ar­range­ment:and the leaves have/are:and the bark is/has:and the twigs:and the fruit (if present):then you have:
tree╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood  genus Cornus.
sub-opposite ▴▴▴ small teeth dark, lots of light-colored lenticels

inner bark=orange (scrape with knife)
often have a terminal thorn are small, black, taste disgusting after a few seconds—spit it out lookalike common buckthorn  Rhamnus cathartica.
╟ alternate ___ smooth or
▴▴▴▴ very small teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn;
hairy
lookalike glossy buckthorn  Frangula alnus.
▲▲ teeth, visible within arm's length dark, lots of light-colored lenticels

if mature, looks covered with burnt potato chips/​crisps
are small, black, taste like 🍒︎ cherry even if sour lookalike ◼︎🍒︎ black cherry  Prunus serotina.
no more than waist-high (<1 m) (<3 ft)▲▲ teeth no hair this plant, please continue below …

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Learn more about alderleaf buckthorn Rhamnus alnifolia

🔍︎ 🔍︎ images Discover Life Encyclopedia of Life Michigan Flora Minnesota Wildflowers Flora of North America NRCS PLANTS db Wikipedia