diff --git a/README.md b/README.md
index 601f2f9..8d02e6a 100644
--- a/README.md
+++ b/README.md
@@ -34,9 +34,13 @@ pipx install pricehist
 - **`gnucash-sql`**: [GnuCash](https://www.gnucash.org/) SQL
 - **`ledger`**: [Ledger](https://www.ledger-cli.org/) and [hledger](https://hledger.org/)
 
-## Examples
+## Usage
 
-Show usage information:
+The following sections demonstrate different uses of `pricehist`.
+
+### Online help
+
+The `-h` option will show usage information:
 
 ```bash
 pricehist -h
@@ -58,7 +62,7 @@ commands:
     fetch                 fetch prices
 ```
 
-Show usage information for the `fetch` command:
+Here's the usage information for the `fetch` command:
 
 ```
 pricehist fetch -h
@@ -94,28 +98,48 @@ optional arguments:
   --fmt-csvdelim CHAR      field delimiter for CSV output (default: ',')
 ```
 
-Fetch prices after 2021-01-04, ending 2021-01-15, as CSV:
+### Fetching prices
+
+Fetch prices by choosing a source, a pair and, optionally, a time interval:
 
 ```bash
-pricehist fetch ecb EUR/AUD -sx 2021-01-04 -e 2021-01-15 -o csv
+pricehist fetch ecb EUR/AUD -s 2021-01-04 -e 2021-01-08
 ```
 ```
 date,base,quote,amount,source,type
+2021-01-04,EUR,AUD,1.5928,ecb,reference
 2021-01-05,EUR,AUD,1.5927,ecb,reference
 2021-01-06,EUR,AUD,1.5824,ecb,reference
 2021-01-07,EUR,AUD,1.5836,ecb,reference
 2021-01-08,EUR,AUD,1.5758,ecb,reference
-2021-01-11,EUR,AUD,1.5783,ecb,reference
-2021-01-12,EUR,AUD,1.5742,ecb,reference
-2021-01-13,EUR,AUD,1.5734,ecb,reference
-2021-01-14,EUR,AUD,1.5642,ecb,reference
-2021-01-15,EUR,AUD,1.568,ecb,reference
 ```
 
-In Ledger format:
+The default output format is CSV, which is suitable for use in spreadsheets and
+with other tools. For example, on the command line using
+[gnuplot](http://www.gnuplot.info/) we can chart Bitcoin prices:
 
 ```bash
-pricehist fetch ecb EUR/AUD -s 2021-01-01 -o ledger | head
+pricehist fetch coindesk BTC/USD -s 2021-01-01 | \
+  cut -d, -f1,4 | \
+  sed 1d | \
+  gnuplot -p -e '
+    set datafile separator ",";
+    set xdata time;
+    set timefmt "%Y-%m-%d";
+    set format x "%b\n%Y";
+    plot "/dev/stdin" using 1:2 with lines title "BTC/USD"
+  '
+```
+
+![BTC/USD prices](example-gnuplot.png)
+
+### Choosing and customizing the output format
+
+You can choose a different output format: `ledger`, `beancount` or
+`gnucash-sql`.
+
+```bash
+pricehist fetch ecb EUR/AUD -s 2021-01-04 -e 2021-01-08 -o ledger
 ```
 ```
 P 2021-01-04 00:00:00 EUR 1.5928 AUD
@@ -123,14 +147,28 @@ P 2021-01-05 00:00:00 EUR 1.5927 AUD
 P 2021-01-06 00:00:00 EUR 1.5824 AUD
 P 2021-01-07 00:00:00 EUR 1.5836 AUD
 P 2021-01-08 00:00:00 EUR 1.5758 AUD
-P 2021-01-11 00:00:00 EUR 1.5783 AUD
-P 2021-01-12 00:00:00 EUR 1.5742 AUD
-P 2021-01-13 00:00:00 EUR 1.5734 AUD
-P 2021-01-14 00:00:00 EUR 1.5642 AUD
-P 2021-01-15 00:00:00 EUR 1.568 AUD
 ```
 
-Generate SQL for a GnuCash database and apply it immediately:
+The formatting options let you control certain details of the output. The
+following example removes the time of day (to make it suitable for hledger) and
+makes a few other adjustments.
+
+```bash
+pricehist fetch ecb EUR/AUD -s 2021-01-04 -e 2021-01-08 -o ledger \
+  --fmt-time '' --fmt-datesep / --fmt-base € --fmt-quote $ --fmt-symbol left
+```
+```
+P 2021/01/04 € $1.5928
+P 2021/01/05 € $1.5927
+P 2021/01/06 € $1.5824
+P 2021/01/07 € $1.5836
+P 2021/01/08 € $1.5758
+```
+
+### GnuCash SQL output
+
+You can generate SQL for a GnuCash database and apply it immediately with one
+of the following commands:
 
 ```bash
 pricehist fetch ecb EUR/AUD -s 2021-01-01 -o gnucash-sql | sqlite3 Accounts.gnucash
@@ -138,12 +176,197 @@ pricehist fetch ecb EUR/AUD -s 2021-01-01 -o gnucash-sql | mysql -u username -p
 pricehist fetch ecb EUR/AUD -s 2021-01-01 -o gnucash-sql | psql -U username -d databasename -v ON_ERROR_STOP=1
 ```
 
-## Design choices
+Beware that the GnuCash project itself does not support integration at the
+database level, so there is a risk that the SQL generated by `pricehist` will
+be ineffective or even damaging for some new version of GnuCash.
 
-To keep things simple, at least for now, `pricehist` provides only univariate
-time series of daily historical prices. It doesn't provide other types of
-market, financial or economic data, real-time prices, or other temporal
-resolutions. Multiple or multivariate series require multiple invocations.
+In practice, this strategy has been used successfully by other projects.
+Reviewing the SQL and keeping regular database backups is recommended.
+
+### Source information
+
+Some basic information about each source is available:
+
+```bash
+pricehist source ecb
+```
+```
+ID          : ecb
+Name        : European Central Bank
+Description : European Central Bank Euro foreign exchange reference rates
+URL         : https://www.ecb.europa.eu/stats/exchange/eurofxref/html/index.en.html
+Start       : 1999-01-04
+Types       : reference
+```
+
+Symbol information can be listed for most sources, either as full pairs or as
+separate base and quote symbols that work in particular combinations:
+
+```bash
+pricehist source ecb --symbols
+```
+```
+EUR/AUD    Euro against Australian Dollar
+EUR/BGN    Euro against Bulgarian Lev
+EUR/BRL    Euro against Brazilian Real
+EUR/CAD    Euro against Canadian Dollar
+EUR/CHF    Euro against Swiss Franc
+...
+```
+
+It may also be possible to search for symbols:
+
+```bash
+pricehist source alphavantage --search Tesla
+```
+```
+TL0.DEX       Tesla, Equity, XETRA, EUR
+TL0.FRK       Tesla, Equity, Frankfurt, EUR
+TSLA34.SAO    Tesla, Equity, Brazil/Sao Paolo, BRL
+TSLA          Tesla Inc, Equity, United States, USD
+TXLZF         Tesla Exploration Ltd, Equity, United States, USD
+```
+
+### Inspecting source interactions
+
+You can see extra information, including `curl` commands to reproduce each
+request to a source, by adding the verbose option (`--verbose` or `-vvv`):
+
+```bash
+pricehist fetch coindesk BTC/USD -s 2021-01-01 -e 2021-01-05 -vvv
+```
+```
+DEBUG Began pricehist run at 2021-08-12 14:38:26.630357.
+DEBUG Starting new HTTPS connection (1): api.coindesk.com:443
+DEBUG https://api.coindesk.com:443 "GET /v1/bpi/historical/close.json?currency=USD&start=2021-01-01&end=2021-01-05 HTTP/1.1" 200 319
+DEBUG curl -X GET -H 'Accept: */*' -H 'Accept-Encoding: gzip, deflate' -H 'Connection: keep-alive' -H 'User-Agent: python-requests/2.25.1' --compressed 'https://api.coindesk.com/v1/bpi/historical/close.json?currency=USD&start=2021-01-01&end=2021-01-05'
+DEBUG Available data covers the interval [2021-01-01--2021-01-05], as requested.
+date,base,quote,amount,source,type
+2021-01-01,BTC,USD,29391.775,coindesk,close
+2021-01-02,BTC,USD,32198.48,coindesk,close
+2021-01-03,BTC,USD,33033.62,coindesk,close
+2021-01-04,BTC,USD,32017.565,coindesk,close
+2021-01-05,BTC,USD,34035.0067,coindesk,close
+DEBUG Ended pricehist run at 2021-08-12 14:38:26.709428.
+```
+
+You can run a logged `curl` command to see exactly what data is returned by the
+source:
+
+```bash
+pricehist fetch coindesk BTC/USD -s 2021-01-01 -e 2021-01-05 -vvv 2>&1 \
+  | grep 'DEBUG curl' | sed 's/^DEBUG //' | bash | jq .
+```
+```json
+{
+  "bpi": {
+    "2021-01-01": 29391.775,
+    "2021-01-02": 32198.48,
+    "2021-01-03": 33033.62,
+    "2021-01-04": 32017.565,
+    "2021-01-05": 34035.0067
+  },
+  "disclaimer": "This data was produced from the CoinDesk Bitcoin Price Index. BPI value data returned as USD.",
+  "time": {
+    "updated": "Jan 6, 2021 00:03:00 UTC",
+    "updatedISO": "2021-01-06T00:03:00+00:00"
+  }
+}
+```
+
+### Use as a library
+
+While not yet optimized for use as a library, you may find `pricehist`'s source
+classes useful in your own scripts:
+
+```python
+$ python
+Python 3.9.6 (default, Jun 30 2021, 10:22:16)
+[GCC 11.1.0] on linux
+Type "help", "copyright", "credits" or "license" for more information.
+>>> from pricehist.series import Series
+>>> from pricehist.sources.ecb import ECB
+>>> series = ECB().fetch(Series("EUR", "AUD", "reference", "2021-01-04", "2021-01-08"))
+>>> series.prices
+[Price(date='2021-01-04', amount=Decimal('1.5928')), Price(date='2021-01-05', amount=Decimal('1.5927')), Price(date='2021-01-06', amount=Decimal('1.5824')), Price(date='2021-01-07', amount=Decimal('1.5836')), Price(date='2021-01-08', amount=Decimal('1.5758'))]
+```
+
+A subclass of `pricehist.exceptions.SourceError` will be raised for any error.
+
+## Terminology
+
+A **source** is the upstream service that can provide a series of prices.
+
+Each **series** of prices is for one pair and price type.
+
+The [**pair**](https://en.wikipedia.org/wiki/Currency_pair) is made up of a
+base and a quote, each given as a symbol. Sometimes you will give the base
+only, and the quote will be determined with information from the source. The
+available pairs, the symbols used in them and the available price types all
+depend on the particular source used.
+
+The **base** is the currency or commodity being valued. Each price expresses
+the value of one unit of the base.
+
+The **quote** is the unit used to express the value of the base.
+
+A **symbol** is a code or abbreviation for a currency or commodity.
+
+The **prices** in a series each have a date and an amount.
+
+The **amount** is the number of units of the quote that are equal to one unit
+of the base.
+
+Consider the following command:
+
+```bash
+pricehist fetch coindesk BTC/USD --type close
+```
+
+- **`coindesk`** is the ID of the CoinDesk Bitcoin Price Index source.
+- **`BTC`** is the symbol for Bitcoin.
+- **`USD`** is the symbol for the United States Dollar.
+- **`BTC/USD`** is the pair Bitcoin against United States Dollar.
+- **`close`** is the price type for last price of each day.
+
+A BTC/USD price of the amount 29,391.775 can be written as
+"BTC/USD = 29391.775" or "BTC 29391.775 USD", and means that one Bitcoin is
+worth 29,391.775 United States Dollars.
+
+## Initial design choices
+
+To keep things simple, `pricehist` provides only univariate time series of
+daily historical prices. It doesn't provide other types of market, financial or
+economic data, real-time prices, or other temporal resolutions. Multiple or
+multivariate series require multiple invocations.
+
+## Future potential features
+
+While the following features are relatively low value or beyond `pricehist`'s
+primary use case, they could be built upon the foundation it has established.
+
+- **Time of day**: Sources sometimes provide specific times for each day's
+  high/low prices and these could be preserved for output. This would require
+  changes to how dates are handled internally, clarification of time zone
+  handling and extension of the time formatting option.
+- **Alternate resolutions**: Some sources can provide higher or lower
+  resolution data, such as hourly or weekly. These could be supported where
+  available. For other cases an option could be provided for downsampling data
+  before output.
+- **Real-time prices**: These generally come from different soruce endpoints
+  than the historical data. Real-time prices will usually have a different
+  price type, such as `last`, `bid` or `ask`. Support for real-time prices
+  would allow adding sources that don't provide historical data. Start and end
+  times are irrelevant when requesting real-time prices. A "follow" option
+  could continuously poll for new prices.
+- **Related non-price data**: Trading volume, spreads, split and dividend
+  events and other related data could be supported. The base/quote/type model
+  used for prices would work for some of this. Other things may require
+  extending the model.
+- **Multivariate series**: Would allow, for example, fetching
+  high/low/open/close prices in a single invocation.
+- **`format` command**: A command for rewriting existing CSV data into one of
+  the other output formats.
 
 ## Alternatives
 
diff --git a/example-gnuplot.png b/example-gnuplot.png
new file mode 100644
index 0000000..f73ca28
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