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:
no more than waist-high (<1 m) (<3 ft)╟ alternate ▲▲ teeth no hair lookalike alderleaf buckthorn  Rhamnus alnifolia
tree▲▲ 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
___ smooth or
very small ▲▲ teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn;
hairy
lookalike glossy buckthorn  Frangula alnus
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 this plant, please continue below …
╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood  genus Cornus

Verify with Google images.

Learn more about common buckthorn Rhamnus cathartica

Management and Control (Wisconsin)  Adobe Acrobat Reader file Best Control Practices BCP (Michigan)  Adobe Acrobat Reader file Best Control Practices BCP (Ontario)  Adobe Acrobat Reader file and its Technical Bulletin  Adobe Acrobat Reader file Fact sheet (Ontario) 🇫🇷 en français  Adobe Acrobat Reader file Control (Minnesota)
Native alternatives (Missouri Botanical Garden).

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