The rule
When to express
Express when the recipe wants citrus aroma without more acid — Old Fashioneds, Martinis, Negronis, Sazeracs. Juice belongs in the shaker; oil belongs on top.
Skill
Squeeze a citrus peel over the drink so the oils spray onto the surface, then often wipe the rim.

The big idea
Expressing a peel sprays aromatic oils onto the surface — perfume before the first sip, not extra juice in the glass.
Keep these
Pinch the peel skin-side down over the drink so oils mist the surface.
Minimize pith — bitter white pith muddies stirred drinks.
Match fruit to drink: orange with whiskey/vermouth, lemon with gin, grapefruit with agave.
A rim wipe puts oil where the lip meets the glass.
The rule
Express when the recipe wants citrus aroma without more acid — Old Fashioneds, Martinis, Negronis, Sazeracs. Juice belongs in the shaker; oil belongs on top.
Cut a thin peel
Wide strip, minimal pith — you're after oils, not bitterness.
Squeeze oils over drink
Pinch peel skin-side down over the glass, then drop or discard.
Optional wipe on rim
A light pass leaves aroma without flooding the sip with juice.
Cut a thin, wide strip of peel. Hold it skin-down a few inches above the drink and pinch. You should see a fine mist on the surface (or smell it immediately).
Optionally wipe the peel around the rim, then drop it in or discard. If the recipe says no garnish, the mist alone is enough.
In the glass
Open a recipe, focus on this move once, then make it again next week.

Whiskey
Express orange (or lemon) over the surface, then wipe the rim. Smell the drink before you sip — the oil should hit first.

Gin
A thin lemon peel, skin-side down over the glass. If it smells like pith and juice, the peel was too thick.

Whiskey
Lemon oil over an absinthe-rinsed glass. The aroma stack is rinse plus peel — neither should drown the rye.

Gin
Orange oil flatters sweet vermouth and gin. Express, then drop or discard — don’t add a thick wedge of pith.