How to read one of these boxes
The six vertical lines are the strings, low E on the left. The thick line across the top is the nut. A dot is a finger; a bar across several strings is one finger laid flat over them, which is what a barre is and why it is drawn as a bar rather than as four separate dots. Above the nut, a circle means the string rings open and a cross means it is not played at all.
When the shape sits higher up the neck there is no nut to draw, so the box shows a plain fret with a number beside it — that is the fret the window starts on, not the fret your first finger goes to. The line of numbers under each box is the same shape written out the way a chord chart prints it: one cell per string, low to high, x for a string you leave alone.
Why these shapes and not others
A guitar has too many valid fingerings for any chord to make a list of them useful. What is on this page is the one a player reaches for: an open shape at the nut where the chord has one — C, G, D, A, E and their sevenths — and otherwise the movable form that sits lowest on the neck. That last rule is what keeps a Bb at the first fret instead of the tenth, where an A form and an E form both technically answer.
None of the fret numbers here were copied out of a book. Each shape is checked against the notes it actually sounds: nothing outside the chord, and every note that makes the chord what it is present. So the claim these boxes make is «this is a voicing of the right chord», which is a stronger thing to be able to say than «this is what page 12 printed».
There is more than one way to play any of them
One box per chord is a chart, not the truth. Nearly every entry above has two or three other shapes behind it — the E form and the A form of the same barre, the open voicing and the one further up — and which of them you want depends on the chord you are coming from and the one you are going to.
That is a choice a page like this cannot make for you, so Strumfolio lets you make it in the song: tap a chord on the sheet and every shape it has is there to page through, and the one you pick is remembered for that chord in that song on every device you open it on.
A ukulele is a different chart
Not a shorter version of this one. Four strings and a four-fret reach leave so few valid voicings that the compact one usually is the one everybody plays, and the chords that are hard swap places completely: E is a stretch on a guitar and three fingers on a ukulele, while Bb is the reverse. The ukulele shapes are on their own chart, or use the switch at the top of this one.
What the app adds
This page is a reference: it knows every chord and nothing about your songs. What Strumfolio does is put the same shapes inside the sheet you are reading on stage — a chord is a tap away from its fingering, the sheet is already in the key you sing in and on the fret you capo at, and all of it works with the phone in aeroplane mode.
If a chord above is one you keep having to look up, the honest fix is usually a capo or a different key rather than more practice — and both of those are free pages here too.