diff --git a/tsdb/chunkenc/histogram.go b/tsdb/chunkenc/histogram.go
index 39bbbf221..e68677129 100644
--- a/tsdb/chunkenc/histogram.go
+++ b/tsdb/chunkenc/histogram.go
@@ -27,8 +27,6 @@ const ()
 // HistogramChunk holds encoded sample data for a sparse, high-resolution
 // histogram.
 //
-// TODO(beorn7): Document the layout of chunk metadata.
-//
 // Each sample has multiple "fields", stored in the following way (raw = store
 // number directly, delta = store delta to the previous number, dod = store
 // delta of the delta to the previous number, xor = what we do for regular
diff --git a/tsdb/docs/format/chunks.md b/tsdb/docs/format/chunks.md
index 30b9cd6f1..8318e0a54 100644
--- a/tsdb/docs/format/chunks.md
+++ b/tsdb/docs/format/chunks.md
@@ -42,9 +42,9 @@ Notes:
 ## XOR chunk data
 
 ```
-┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┐
-│ num_samples <uint16> │ ts_0 <varint> │ v_0 <float64> │ ts_1_delta <uvarint> │ v_1_xor <varbit_xor> │ ts_n_dod <varbit_ts> │ v_n_xor <varbit_xor> │ ... │
-└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┘
+┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┬──────────────────────┬──────────────────────┬──────────────────┐
+│ num_samples <uint16> │ ts_0 <varint> │ v_0 <float64> │ ts_1_delta <uvarint> │ v_1_xor <varbit_xor> │ ts_2_dod <varbit_ts> │ v_2_xor <varbit_xor> │ ... │ ts_n_dod <varbit_ts> │ v_n_xor <varbit_xor> │ padding <x bits> │
+└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┴──────────────────────┴──────────────────────┴──────────────────┘
 ```
 
 ### Notes:
@@ -55,41 +55,90 @@ Notes:
 * `<varint>` and `<uvarint>` have 1 to 10 bytes each.
 * `ts_1_delta` is `ts_1` – `ts_0`.
 * `ts_n_dod` is the “delta of deltas” of timestamps, i.e. (`ts_n` – `ts_n-1`) – (`ts_n-1` – `ts_n-2`).
-* `<v_n_xor>` is the result of `v_n` XOR `v_n-1`.
+* `v_n_xor` is the result of `v_n` XOR `v_n-1`.
 * `<varbit_xor>` is a specific variable bitwidth encoding of the result of XORing the current and the previous value. It has between 1 bit and 77 bits.
   See [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L220-L253).
 * `<varbit_ts>` is a specific variable bitwidth encoding for the “delta of deltas” of timestamps (signed integers that are ideally small).
   It has between 1 and 68 bits.
   see [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L179-L205).
+* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
+* The chunk can have as few as one sample, i.e. `ts_1`, `v_1`, etc. are optional.
 
 ## Histogram chunk data
 
 ```
-┌──────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬────────────────┐
-│ num_samples <uint16> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ samples <data> │
-└──────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴────────────────┘
+┌──────────────────────┬──────────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬────────────────┬──────────────────┐
+│ num_samples <uint16> │ histogram_flags <1 byte> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ samples <data> │ padding <x bits> │
+└──────────────────────┴──────────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴────────────────┴──────────────────┘
 ```
 
 ### Positive and negative spans data:
 
 ```
-┌───────────────────┬────────────────────────┬───────────────────────┬─────┬──────────────────────────┬─────────────────────────┐
-│ num <varbit_uint> │ length_1 <varbit_uint> │ offset_1 <varbit_int> │ ... │ length_num <varbit_uint> │ offset_num <varbit_int> │
-└───────────────────┴────────────────────────┴───────────────────────┴─────┴──────────────────────────┴─────────────────────────┘
+┌─────────────────────────┬────────────────────────┬───────────────────────┬────────────────────────┬───────────────────────┬─────┬────────────────────────┬───────────────────────┐
+│ num_spans <varbit_uint> │ length_0 <varbit_uint> │ offset_0 <varbit_int> │ length_1 <varbit_uint> │ offset_1 <varbit_int> │ ... │ length_n <varbit_uint> │ offset_n <varbit_int> │
+└─────────────────────────┴────────────────────────┴───────────────────────┴────────────────────────┴───────────────────────┴─────┴────────────────────────┴───────────────────────┘
 ```
 
 ### Samples data:
 
 ```
-TODO
+┌──────────────────────────┐
+│    sample_0 <data>       │
+├──────────────────────────┤
+│    sample_1 <data>       │
+├──────────────────────────┤
+│    sample_2 <data>       │
+├──────────────────────────┤
+│          ...             │
+├──────────────────────────┤
+│    Sample_n <data>       │
+└──────────────────────────┘
+```
+
+#### Sample 0 data:
+
+```
+┌─────────────────┬─────────────────────┬──────────────────────────┬───────────────┬───────────────────────────┬─────┬───────────────────────────┬───────────────────────────┬─────┬───────────────────────────┐
+│ ts <varbit_int> │ count <varbit_uint> │ zero_count <varbit_uint> │ sum <float64> │ pos_bucket_0 <varbit_int> │ ... │ pos_bucket_n <varbit_int> │ neg_bucket_0 <varbit_int> │ ... │ neg_bucket_n <varbit_int> │
+└─────────────────┴─────────────────────┴──────────────────────────┴───────────────┴───────────────────────────┴─────┴───────────────────────────┴───────────────────────────┴─────┴───────────────────────────┘
+```
+
+#### Sample 1 data:
+
+```
+┌────────────────────────┬───────────────────────────┬────────────────────────────────┬──────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┐
+│ ts_delta <varbit_uint> │ count_delta <varbit_uint> │ zero_count_delta <varbit_uint> │ sum_xor <varbit_xor> │ pos_bucket_0_delta <varbit_int> │ ... │ pos_bucket_n_delta <varbit_int> │ neg_bucket_0_delta <varbit_int> │ ... │ neg_bucket_n_delta <varbit_int> │
+└────────────────────────┴───────────────────────────┴────────────────────────────────┴──────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┘
+```
+
+#### Sample 2 data and following:
+
+```
+┌─────────────────────┬────────────────────────┬─────────────────────────────┬──────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┐
+│ ts_dod <varbit_int> │ count_dod <varbit_int> │ zero_count_dod <varbit_int> │ sum_xor <varbit_xor> │ pos_bucket_0_dod <varbit_int> │ ... │ pos_bucket_n_dod <varbit_int> │ neg_bucket_0_dod <varbit_int> │ ... │ neg_bucket_n_dod <varbit_int> │
+└─────────────────────┴────────────────────────┴─────────────────────────────┴──────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┘
 ```
 
 ### Notes:
 
+* `histogram_flags` is a byte of which currently only the first two bits are used:
+  * `10`: Counter reset between the previous chunk and this one.
+  * `01`: No counter reset between the previous chunk and this one.
+  * `00`: Counter reset status unknown.
+  * `11`: Chunk is part of a gauge histogram, no counter resets are happening.
 * `zero_threshold` has a specific encoding:
   * If 0, it is a single zero byte.
   * If a power of two between 2^-243 and 2^10, it is a single byte between 1 and 254.
   * Otherwise, it is a byte with all bits set (255), followed by a float64, resulting in 9 bytes length.
 * `schema` is a specific value defined by the exposition format. Currently valid values are -4 <= n <= 8.
 * `<varbit_int>` is a variable bitwidth encoding for signed integers, optimized for “delta of deltas” of bucket deltas. It has between 1 bit and 9 bytes.
+  See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L31-L60).
 * `<varbit_uint>` is a variable bitwidth encoding for unsigned integers with the same bit-bucketing as `<varbit_int>`.
+  See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L136-L165).
+* `<varbit_xor>` is a specific variable bitwidth encoding of the result of XORing the current and the previous value. It has between 1 bit and 77 bits.
+  See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/histogram.go#L538-L574).
+* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
+* Note that buckets are inherently deltas between the current bucket and the previous bucket. Only `bucket_0` is an absolute count.
+* The chunk can have as few as one sample, i.e. sample 1 and following are optional.
+* Similarly, there could be down to zero spans and down to zero buckets.