To use a tree twig key means learning a twig's botanical parts. A key can help you identify a tree to the specific species by asking two question where you can affirm one and eliminate the other. This is called a dichotomous key.
Here are several of the best online twig keys:
Terms You Must Know!
Opposite or Alternate Twigs: Most tree twig keys start with the arrangement of leaf, limb and buds. It is the primary first separation of the most common tree species. You can eliminate major blocks of trees just by observing its leaf and twig arrangement.
Alternate leaf attachments have one unique leaf at each leaf node and typically alternate direction along the stem. Opposite leaf attachments pair leaves at each node. Whorled leaf attachment is where three or more leaves attach at each point or node on the stem.
The opposites are maple, ash, dogwood, paulownia buckeye and boxelder (which is really a maple). The alternates are oak, hickory, yellow poplar, birch, beech, elm, cherry, sweetgum and sycamore.
The Terminal Bud: There is a bud on the tip of every twig where growth occurs. It is often larger than the lateral buds and some can be absent. Trees easily identified by their terminal buds are yellow poplar (mitten or duckbilled shaped), dogwood (clove-shaped flower bud) and oak (clustered bud ends).
The Lateral Buds: These are buds on each side of the branch. The trees easily identified by a lateral bud is beech (long, pointed scaled bud) and elm (buds off center over leaf scar).
The Leaf Scar: This is a scar of leaf attachment. When the leaf drops, a scar is left just under the bud and it can be unique. The trees easily identified by its leaf scars are hickory (3-lobed), ash (shield shaped)and dogwood (leaf scar encircles the twig).
The Lenticel: There are cork-filled pores on most trees that permit the living inner bark to breath. I use the narrow, long and light lenticels to partly identify just one species that can be tricky - black cherry.
The Bundle Scar: You can see scars within the leaf scar that are a big help in identification. These visible dots or lines are cork filled ends of tubes that supply the leaf with water. The trees easily identified by its bundle or vein scars are ash (continuous bundle scars), maple (three bundle scars), and oaks (numerous scattered bundle scars)
The Stipule Scar: This is the scar of a leaf-like attachment just off the leaf stem. Since all trees do not have stipules the presence or absence of stipule scars is often helpful in identifying a winter twig. The trees easily identified by its stipule scar is magnolia and yellow poplar.
The Pith: The pith is the soft inner core of the twig. The trees easily identified by its pith are black walnut and butternut (both with chambered pith) and hickory (tan, 5-sided pith).
One bit of caution when using the above markers. You need to observe an average-looking and maturing tree and stay away from root sprouts, seedlings, suckers and juvenile growth. Rapidly growing young growth can (but not always) have atypical markers that will confuse the beginning identifier.