Went to tune a wonderful old Steinway upright today. Noticed a problem with the hammers 'blocking' against the strings. This is a sure sign that the 'key dip' is excessive. Key dip is the amount the key goes down when played (should be around 3/8" or 10mm). Keys were dipping way more than that on this piano. I removed a few keys to find the felts completely disintegrated (photo above). Each pin should have a neat green disc of felt around it, about 2cm in diameter, as shown below.