Stack, reach, effective top tube and standover in plain terms, why small hybrid frames often keep a long front end, and how to check a shop bike with a level and a tape.
A shop will hand you the smallest frame on the floor, watch you straddle it, ask whether it feels alright, and treat a clear standover gap as proof of fit. Standover answers one question only: whether you can stand over the bike without injury. It says nothing about the distance from the saddle to the handlebar, which is the measurement that decides whether an hour on the bike leaves your neck and hands tired. Plenty of riders on correctly labeled small frames are riding something too long for them, and the size chart never flagged it.
The four numbers, in plain terms
Stack is the vertical distance from the bottom bracket to the top of the head tube. Reach is the horizontal distance between those same two points. Together they describe where the front of the bike sits relative to where you pedal, and they do not change when someone swaps the seatpost or slides the saddle. Effective top tube is the horizontal distance from the head tube to an imaginary point above the bottom bracket at seat height, which matters because most hybrid frames have a sloping top tube and the actual tube length tells you little. Standover is simply the height of the frame where you straddle it.
Stack and reach are the honest pair. Effective top tube is useful for comparing two bikes with similar seat angles and misleading otherwise, since a slacker seat tube pushes that measurement out without moving the bars any farther away from a rider sitting where the saddle actually needs to be. The National Institute of Standards and Technology is the federal body responsible for measurement standards, and the reason bicycle sizing feels slippery is that no equivalent discipline is applied to it. Manufacturers publish what they choose, in the way they choose.
Why the small sizes stay long
Frames are scaled, not redesigned. When a maker builds a size run from extra small to extra large, the seat tube shortens fast, because standover is the number buyers check in a parking lot, and it is the easy one to fix. The top tube shortens slowly, because a short front-center brings the front wheel back toward the pedals, and toe overlap follows: your shoe clips the tire when you turn at low speed. Wheel size sets a floor on how short the front end can go. A 700c hybrid in a nominal 13 or 14 inch frame frequently carries a reach figure barely shorter than the size above it.
The result is a bike with a low, easily cleared top tube and a cockpit built for a taller torso. That is a solvable problem, and knowing which of the four numbers is out of range tells you whether it is solvable cheaply. Stack too low is fixed with spacers and a riser bar. Reach too long is fixed with a shorter stem, within limits, and a bar with less forward sweep. Reach too long by four centimeters is not a stem problem, and no amount of saddle-forward adjustment substitutes.
Measuring the bike in the shop
Bring a tape measure and a small spirit level, or a level app on a phone with a straight edge to rest it on. Ask to have the bike held upright, or lean it against a wall with both tires touching the floor. For reach, hold the level horizontally against the center of the head tube at its top edge, run it back until it sits above the bottom bracket, and measure straight down to the bottom bracket center. The horizontal run of the level is your reach. For stack, measure that vertical drop.
Effective top tube uses the same method: level from the top of the head tube back to the seatpost, and read the horizontal distance. Two or three tries will get you within half a centimeter, which is close enough to compare bikes honestly. Write the numbers down with the model and size, because catalog geometry tables are sometimes measured differently from what is bolted together in front of you, and a bike you already ride comfortably gives you a reference figure that every subsequent measurement can be judged against.
What a careful reader does with the numbers
Compare candidate bikes on reach first, then stack, then look at effective top tube only to sanity-check the pair. A frame whose reach is within a centimeter or two of a bike you already like is worth a test ride even if the label says a size you have never ridden. A frame whose reach is long by more than about three centimeters will need a stem so short that the steering goes quick and nervous, and that is the point at which a different model, or a different wheel size, is the better purchase.
The measurements take ten minutes and they travel between shops in a notebook. That is the whole advantage: you stop negotiating over a letter on a sticker and start comparing two specific bicycles by the dimensions that determine how they will feel at mile fifteen.