I have synesthesia but it’s about as mild as you can get. I can hear colours and I can feel numbers, but only when my senses are close to deprivation (the former in particular is more of a change of background white noise than anything else).
It’s becoming clear with more research that about 1 in 10 have synesthesia though, and the majority of those cases are far more interesting than mine I bet. So tell us all about your synesthesia!
My friend has something calledSpatial Sequence Synesthesia, so for him anything in a sequence (numbers, dates, days of the week etc) has a spatial location. He was explaining to me once that he sees the numbers going straight until 5, then they turn left until 20 etc. Dunno if that’s more interesting than yours or not. Edit: not sure how different this is from Number form
What is interesting is that he thought everyone else was like that until quite recently. He happened upon an article about it on the net, and all the commenters seemed to think it was weird and didn’t know what the article was on about, and so he asked his wife and son about it and to his surprise neither of them had specific locations for sequences, he thought everyone would just have different locations, and thinks we’re all weird for not having them lol.
I’ve mentioned on the SDMB before that in my head, letters, numbers, and colors all have specific genders. I remember being very confused as a child when people didn’t intrinsically understand that red was a girl. Some of these associations have weakened somewhat over the years, but most are still as strong as ever.
Interestingly, many states and countries have gender too, but they’re all or almost all male.
–Cliffy
Do you see a pattern with which numbers are male vs. female? I have a very weak association between odd = male, even = female, with 10, 100, 1,000, and 1,000,000 being exceptions.
I’m the same way. Numbers, hours of the day, days of the week, months, years, centuries . . . I have different ways of picturing each. For example, years are white on a black background, beginning small, in the upper right, and gradually getting larger, moving in a long arc toward the lower left. Days of the week and months all have their own color and spatial relationship.
Also: Each letter and number has a specific color. And musical notes and keys have different colors as well. Ages too. Now, I don’t actually see these colors, but I know what they are. It’s like looking at a grayscale image. I know that the sky is blue and the grass is green.
Holy jumping mackerels, that has a name? I thought it was just another one of my bizarre brain things. If I’m reading the article right, then yes, it’s different from the ‘number form’ one, in the same sense that learning a route by walking it is different than learning a route from a map. If the spatial sequence thing is what I have, then the “spatial” part of it is not like “arranged on a piece of paper”. It’s “having the proprioceptive and/or kinesthetic sense that you are occupying a space, in which this sequence is also embedded”. It’s not just that the next term in the sequence is to the left of something else, it’s the feeling that getting there involves you personally turning to the left in some imaginary volume of space.
I don’t know if it’s linked, but tones are also very strongly tied to space for me. I sing, and for me a higher pitch should be located higher in space, specifically closer to the top of my head. I tried to teach myself guitar once, and it was a disaster – guitars are exactly backwards, with the bass string higher off the floor. Piano I can kind of do, but the left-to-right thing doesn’t feel natural. I can tune things, but only to similar waveforms; I could tune the guitar to a guitar or to a piano or probably to a voice, but the sine wave guitar tuner was useless. I also never had any idea whether I needed to make the pitch “higher” or “lower” to match, I just knew which way to turn the tuning peg. And if I thought about it too hard, I couldn’t do it at all.
The article also has a note on memories. I have an eidetic-ish memory, and always have. I can’t do the superimposed dot pattern tests, and I can’t usually read text off of recalled pages, but I’m apparently very scary at recalling rooms and video clips and general arrangements and so forth. Motion, or the sense of space, is a large part of it.
I find this is also true. I live in this head, and it’s always looked like that. It took me many years to develop a general sense of what not-normal things I do, and when I should not tell people I’m doing them. 
Missed the edit window. (Second time tonight. Urf.)
It occurs to me that I’m also chronically pareidolic. I actually just wrote about it the other night. I wonder if it’s linked? I’ve always sort of thought of it as a very low-grade “everything”-> “everything” synesthesia-analogue, but I also always figured that was my idiosyncratic way of looking at it.
I guess I have spatial sequence synesthesia, since I do visualize numbers and especially the calendar. It makes it easy for me to put things in historical context.
I used to think that numbers has personalities; seven was a nasty character.
And it’s yellow.
Nope. 0,1,3,4, 7 and 10 are all male. The rest are girls except 2, which is male, but it’s a very weak association. Larger numbers are pretty much always the same gender as the last digit of the number. There’s no rhyme or reason to the letters at all.
I sort of realized when I was a teenager that other people didn’t know what sex all the letters and numbers were, but I still didn’t really make the connection that it was in my head and not actually true until I was older and first encountered people talking about synesthesia. To the extent I’ve accepted it at all, that is. Mostly I still think all the rest of you people just don’t see what is obviously there.
–Cliffy
Guitar chord shapes have colors. It’s not the actual sound of the chords, just the shapes as fingered - the colors stay the same when I play with a capo. The major chords are the brightest colors: A is blue, B is orange, C is yellow, D is red, E is white, F is green, G is purple. It carries over to most of the minor and seventh chords as well, with a twist: Am is a very light blue, Dm stays red but a much brighter shade, Bm is a peachy sort of orange. Fm is a darker green, for some reason, and B7 is brown. Not all shapes have them, though. Interestingly enough, the barre chords mostly get the same colors as the open chords - the D barre chord is still red, for instance.
I only really noticed that I was doing this when I was practicing a song and kept reminding myself: “no, play the red chord at this point”, as a mnemonic, because it matched a line in the lyrics.
I have your basic color grapheme synesthesia–my letters and numbers have color. I’ve had it since I was a kid but didn’t find out what it was called (or even that it was a thing) till I got out of college.
Very mild in my case - I see a calendar year as egg-shaped. Each year is contained in it’s own egg and has no visible relation to a year before or after.
I used to drive people crazy by telling them I’d do something “tomorrow” on a Friday, and mean I’d do it Monday. All of the school days were in one long continuous string, with the weekends snipped off and dumped loose in a pile somewhere else. I still visualize them that way, although to a lesser degree, as I now do freelance work that respects no schedule. 
The sheer number of different visualizations and descriptions of all these things are fascinating. I ask people about this in real life, and they look at me like I’m mad. I wonder if there are any recurring themes?
I feel and see sound, due to the fact that I am deaf. The type of loss I have means I feel through bone conduction
In certain limited (and quite minor) circumstances, I see sounds as flashes of light.
This happens when I’m partially falling asleep, or at least rather tired, with my eyes closed, in a darkened room. Any time there is any sudden and abrupt (not necessarily loud) sound, I see a definite flash of white light with it. Any sharp knocking sound (like the knocks a house makes as it cools and settles for the night) will do. Even the quiet “ping” sound that incandescent light bulbs make as they cool will do. Once upon a time, I assumed that light bulbs actually gave off a quick flash of light when that happened, and I wondered mightily what the physics behind that could possibly be.
Three is yellow, seven is blue. Duh.
I do this with a calendar. It’s a circle…ish. Looks almost like an app. I’ve never been able to explain it and never thought to draw it. A week is a strip. It’s all 3D.
<shrug>
My strongest and most consistent types are colored graphemes and colored music. And yes, a few letters and numbers have personalities (when I was in second grade I insisted on drawing sad faces in my 8’s to show how much I hated that number).
That last link is interesting – it made me realize that I have extremely weak gender associations with characters of the Cyrillic alphabet, which I learned as a teenager. The ones that are the same as Phonenican letters are the same sex, I guess. Some of the others are almost genderless (although I think I can “see” their gender if I concentrate).
–Cliffy